Qt version :
qt-4.5.0-14.fc11.x86_64
qt-4.5.0-14.fc11.i586

Here using:

Qt: 4.5.1 (qt-copy  958974)
KDE: 4.2.85 (KDE 4.2.85 (KDE 4.3 Beta1))
kdelibs svn rev. 967040 / kdebase svn rev. 967041
on ArchLinux i686 - Kernel 2.6.29.2

I can reproduce the crash with the provided steps.
However, the "Overwrite" file dialog didn't appear. 
It only appeared a message in the Dolphin statusbar saying "The file X already exists"

And the notification can be cancelled and removed (this was fixed from the Plasma side)

I guess it is trying to kill a job which doesn't exist. I wonder if this is a kdelibs bug.

Backtrace:
---

Application: Dolphin (dolphin), signal: Segmentation fault
[KCrash Handler]
#6  0x0070004f in ?? ()
#7  0xb7317d32 in KJob::kill (this=0x874d598, verbosity=KJob::EmitResult) at /home/kde-devel/kde/src/KDE/kdelibs/kdecore/jobs/kjob.cpp:106
#8  0xb76c9a7f in KUiServerJobTracker::Private::_k_killJob (this=0x8795438) at /home/kde-devel/kde/src/KDE/kdelibs/kdeui/jobs/kuiserverjobtracker.cpp:58
#9  0xb76c9af9 in KUiServerJobTracker::qt_metacall (this=0x87452e0, _c=QMetaObject::InvokeMetaMethod, _id=25, _a=0xbfba6a98) at /home/kde-devel/kde/build/KDE/kdelibs/kdeui/kuiserverjobtracker.moc:94
#10 0xb67e3b3b in QMetaObject::activate (sender=0x87393c8, from_signal_index=5, to_signal_index=5, argv=0x0) at kernel/qobject.cpp:3117
#11 0xb67e4f07 in QMetaObject::activate (sender=0x87393c8, m=0xb78d3a64, local_signal_index=0, argv=0x0) at kernel/qobject.cpp:3194
#12 0xb783e127 in OrgKdeJobViewInterface::cancelRequested (this=0x87393c8) at /home/kde-devel/kde/build/KDE/kdelibs/kdeui/jobviewiface.moc:113
#13 0xb783e52b in OrgKdeJobViewInterface::qt_metacall (this=0x87393c8, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0xbfba6cc0) at /home/kde-devel/kde/build/KDE/kdelibs/kdeui/jobviewiface.moc:90
#14 0xb72020be in QDBusConnectionPrivate::deliverCall (this=0x82f9810, object=0x87393c8, msg=@0x87398ac, metaTypes=@0x87398b0, slotIdx=5) at qdbusintegrator.cpp:891
#15 0xb720d4d8 in QDBusCallDeliveryEvent::placeMetaCall (this=0x8739880, object=0x87393c8) at qdbusintegrator_p.h:101
#16 0xb67e19ef in QObject::event (this=0x87393c8, e=0x8739880) at kernel/qobject.cpp:1115
#17 0xb69f51d5 in QApplicationPrivate::notify_helper (this=0x83011c8, receiver=0x87393c8, e=0x8739880) at kernel/qapplication.cpp:4057
#18 0xb69f5589 in QApplication::notify (this=0xbfba764c, receiver=0x87393c8, e=0x8739880) at kernel/qapplication.cpp:3604
#19 0xb76cf24a in KApplication::notify (this=0xbfba764c, receiver=0x87393c8, event=0x8739880) at /home/kde-devel/kde/src/KDE/kdelibs/kdeui/kernel/kapplication.cpp:307
#20 0xb67cd6cb in QCoreApplication::notifyInternal (this=0xbfba764c, receiver=0x87393c8, event=0x8739880) at kernel/qcoreapplication.cpp:606
#21 0xb69f1bbd in QCoreApplication::sendEvent (receiver=0x87393c8, event=0x8739880) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
#22 0xb67cdbb3 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x82c9998) at kernel/qcoreapplication.cpp:1244
#23 0xb67cdd6f in QCoreApplication::sendPostedEvents (receiver=0x0, event_type=0) at kernel/qcoreapplication.cpp:1140
#24 0xb6aa45ec in QCoreApplication::sendPostedEvents () at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:218
#25 0xb67fd035 in postEventSourceDispatch (s=0x82c94c0) at kernel/qeventdispatcher_glib.cpp:209
#26 0xb5fe9718 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#27 0xb5fecc7b in g_main_context_iterate () from /usr/lib/libglib-2.0.so.0
#28 0xb5fecdf8 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#29 0xb67fc41e in QEventDispatcherGlib::processEvents (this=0x83099c0, flags={i = -1078299384}) at kernel/qeventdispatcher_glib.cpp:324
#30 0xb6aa2dd8 in QGuiEventDispatcherGlib::processEvents (this=0x83099c0, flags={i = -1078299336}) at kernel/qguieventdispatcher_glib.cpp:202
#31 0xb67ca6b0 in QEventLoop::processEvents (this=0xbfba75c0, flags={i = -1078299264}) at kernel/qeventloop.cpp:149
#32 0xb67ca90a in QEventLoop::exec (this=0xbfba75c0, flags={i = -1078299192}) at kernel/qeventloop.cpp:196
#33 0xb67cdea3 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
#34 0xb69f4eee in QApplication::exec () at kernel/qapplication.cpp:3526
#35 0x0807a6af in main (argc=5, argv=0xbfba7824) at /home/kde-devel/kde/src/KDE/kdebase/apps/dolphin/src/main.cpp:94

sorry for being a bit misleading, the statement "dolphin claims
that file already exists" i was actually referring to the status bar of dolphin.

i've found a way to make the pop up disappear, which is to drag them away and make them become a "widget" and then close it there.

Here's a better backtrace from KDE 4.3.2:
(https://launchpad.net/bugs/453651)

.
Thread 3 (process 4573):
#0  0x007d5422 in __kernel_vsyscall ()
No symbol table info available.
#1  0x06fbd142 in pthread_cond_timedwait@@GLIBC_2.3.2 ()
   from /lib/tls/i686/cmov/libpthread.so.0
No symbol table info available.
#2  0x083bf7e4 in __pthread_cond_timedwait (cond=0xa104b70, mutex=0xa104b58, 
    abstime=0xb5ec2330) at forward.c:152
	__p = (int (*)(pthread_cond_t *, pthread_mutex_t *, 
    const struct timespec *)) 0xfffffdfc
#3  0x081089ff in metronom_sync_loop (this=0xa104b10) at metronom.c:870
	tv = {tv_sec = 1255743222, tv_usec = 910107}
	ts = {tv_sec = 1255743227, tv_nsec = 910107000}
	scr = <value optimized out>
	pts = 581115262953857840
#4  0x06fb880e in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
No symbol table info available.
#5  0x083b27ee in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:130
No locals.
.
Thread 2 (process 4679):
#0  0x007d5422 in __kernel_vsyscall ()
No symbol table info available.
#1  0x083ab981 in select () from /lib/tls/i686/cmov/libc.so.6
No locals.
#2  0x0351baf5 in QProcessManager::run (this=0x9bd0d38)
    at io/qprocess_unix.cpp:296
	readset = {fds_bits = {64, 0 <repeats 31 times>}}
	nselect = <value optimized out>
	c = 0 '\0'
#3  0x0344ce32 in QThreadPrivate::start (arg=0x9bd0d38)
    at thread/qthread_unix.cpp:188
	data = (QThreadData *) 0x9bd68f8
#4  0x06fb880e in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
No symbol table info available.
#5  0x083b27ee in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:130
No locals.
.
Thread 1 (process 4472):
#0  0x01053078 in typeinfo name for KTimeZoneSource ()
   from /usr/lib/libkdecore.so.5
No symbol table info available.
#1  0x00f174b2 in KJob::kill (this=0xa39ee50, verbosity=KJob::EmitResult)
    at ../../kdecore/jobs/kjob.cpp:106
No locals.
#2  0x00aca497 in KUiServerJobTracker::Private::_k_killJob (this=0xa09c348)
    at ../../kdeui/jobs/kuiserverjobtracker.cpp:58
	job = (class KJob *) 0x107c428
#3  0x00aca509 in KUiServerJobTracker::qt_metacall (this=0x9eb5748, 
    _c=QMetaObject::InvokeMetaMethod, _id=25, _a=0xbf82164c)
    at ./kuiserverjobtracker.moc:94
No locals.
#4  0x03553263 in QMetaObject::activate (sender=0x9fca058, 
    from_signal_index=5, to_signal_index=5, argv=0x0)
    at kernel/qobject.cpp:3113
	c = <value optimized out>
	method = 25
	currentSender = {sender = 0x9fca058, signal = 5, ref = 1}
	previousSender = (QObjectPrivate::Sender *) 0x0
	i = 0
	count = 1
	signal = 5
	empty_argv = {0x0}
	locker = {{mtx = 0x9bade7c, val = 163241596}}
	currentThreadData = <value optimized out>
	connectionLists = (QObjectConnectionListVector *) 0x9c3cff8
#5  0x03553ec2 in QMetaObject::activate (sender=0x9fca058, m=0xc87604, 
    local_signal_index=0, argv=0x0) at kernel/qobject.cpp:3187
	signal_index = 5
#6  0x00c3c5a7 in OrgKdeJobViewInterface::cancelRequested (this=0x9fca058)
    at jobviewiface.moc:113
No locals.
#7  0x00c3c9a3 in OrgKdeJobViewInterface::qt_metacall (this=0x9fca058, 
    _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0xbf82194c)
    at jobviewiface.moc:90
No locals.
#8  0x0062d7b4 in QDBusConnectionPrivate::deliverCall (this=0x9bc43b0, 
    object=0x9fca058, msg=@0xa3ce36c, metaTypes=@0xa3ce370, slotIdx=5)
    at qdbusintegrator.cpp:891
	context = {connection = {static staticMetaObject = {d = {
        superdata = 0x0, stringdata = 0x66aec0 "QDBusConnection", 
        data = 0x66b0a0, extradata = 0x0}}, d = 0x9bc43b0}, 
  message = @0xa3ce36c}
	old = (QDBusContextPrivate *) 0x0
	ptr = {o = 0x9fca058}
	params = {a = 10, s = 1, ptr = 0xbf82194c, {
    array = "\000\000\000\000\000\000\000\000¨\2006\n¨\031\202¿vmf\000°C¼\t\000\000\000\000\a\000\000\000¨\2006\n4Âæ\tð\035\202¿\230\031\202¿", 
    q_for_alignment_1 = 0, q_for_alignment_2 = 0}}
	i = <value optimized out>
	outputArgs = {{p = {static shared_null = {ref = {_q_value = 14725}, 
        alloc = 0, begin = 0, end = 0, sharable = 1, array = {0x0}}, 
      d = 0x80c0830}, d = 0x80c0830}}
	fail = true
	auxParameters = {{p = {static shared_null = {ref = {
          _q_value = 14725}, alloc = 0, begin = 0, end = 0, sharable = 1, 
        array = {0x0}}, d = 0x80c0830}, d = 0x80c0830}}
	pCount = 0
warning: (Internal error: pc 0x635197 in read in psymtab, but not in symtab.)
.
warning: (Internal error: pc 0x635196 in read in psymtab, but not in symtab.)
.
warning: (Internal error: pc 0x635196 in read in psymtab, but not in symtab.)
.
#9  0x00635197 in QDBusCallDeliveryEvent::placeMetaCall (this=warning: (Internal error: pc 0x635196 in read in psymtab, but not in symtab.)
.
warning: (Internal error: pc 0x635160 in read in psymtab, but not in symtab.)
.
warning: (Internal error: pc 0x635196 in read in psymtab, but not in symtab.)
.
0xa3ce340, 
    object=warning: (Internal error: pc 0x635196 in read in psymtab, but not in symtab.)
.
0x9fca058) at qdbusintegrator_p.h:101
warning: (Internal error: pc 0x635196 in read in psymtab, but not in symtab.)
.
No locals.
#10 0x0354d5fe in QObject::event (this=0x9fca058, e=0xa3ce340)
    at kernel/qobject.cpp:1111
	currentSender = {sender = 0x9bc43b0, signal = -1, ref = 1}
	previousSender = (QObjectPrivate::Sender * const) 0x0
#11 0x011a5f54 in QApplicationPrivate::notify_helper (this=0x9bd9518, 
    receiver=0x9fca058, e=0xa3ce340) at kernel/qapplication.cpp:4056
	consumed = <value optimized out>
#12 0x011ad67c in QApplication::notify (this=0xbf82215c, receiver=0x9fca058, 
    e=0xa3ce340) at kernel/qapplication.cpp:3603
	res = true
#13 0x00acfbfa in KApplication::notify (this=0xbf82215c, receiver=0x9fca058, 
    event=0xa3ce340) at ../../kdeui/kernel/kapplication.cpp:302
No locals.
#14 0x0353d6cb in QCoreApplication::notifyInternal (this=0xbf82215c, 
    receiver=0x9fca058, event=0xa3ce340) at kernel/qcoreapplication.cpp:610
	result = false
	cbdata = {0x9fca058, 0xa3ce340, 0xbf821d6f}
	threadData = (QThreadData *) 0x9bade48
	returnValue = <value optimized out>
#15 0x0353e2b2 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, 
    event_type=0, data=0x9bade48)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
	pe = (const QPostEvent &) @0x9e11c68: {receiver = 0x9fca058, 
  event = 0x0, priority = 0}
	e = (class QEvent *) 0xa3ce340
	r = (class QObject *) 0x9fca058
	locker = {{mtx = 0x9bade70, val = 163241584}}
	startOffset = 0
	i = (int &) @0x9bade68: 2
#16 0x0353e47d in QCoreApplication::sendPostedEvents (receiver=0x0, 
    event_type=0) at kernel/qcoreapplication.cpp:1140
	data = <value optimized out>
#17 0x035683ff in postEventSourceDispatch (s=0x9bd5700)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:218
No locals.
#18 0x01a99e78 in g_main_context_dispatch () from /lib/libglib-2.0.so.0
No symbol table info available.
#19 0x01a9d720 in ?? () from /lib/libglib-2.0.so.0
No symbol table info available.
#20 0x01a9d853 in g_main_context_iteration () from /lib/libglib-2.0.so.0
No symbol table info available.
#21 0x0356802c in QEventDispatcherGlib::processEvents (this=0x9bada30, flags=
      {i = 36}) at kernel/qeventdispatcher_glib.cpp:327
	d = (QEventDispatcherGlibPrivate * const) 0x9bd5680
	canWait = true
	result = <value optimized out>
#22 0x01246be5 in QGuiEventDispatcherGlib::processEvents (this=0x9bada30, 
    flags={i = 36}) at kernel/qguieventdispatcher_glib.cpp:202
	d = (QGuiEventDispatcherGlibPrivate * const) 0x9bd9618
	returnValue = 40
#23 0x0353bc79 in QEventLoop::processEvents (this=0xbf8220b4, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece.
)
    at kernel/qeventloop.cpp:149
	d = (QEventLoopPrivate * const) 0x9c61498
#24 0x0353c0ca in QEventLoop::exec (this=0xbf8220b4, flags={i = 0})
    at kernel/qeventloop.cpp:201
	d = (QEventLoopPrivate * const) 0x9c61498
	app = <value optimized out>
#25 0x0353e53f in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
	threadData = (QThreadData *) 0x9bade48
	eventLoop = {<QObject> = {_vptr.QObject = 0x362e7a8, 
    static staticMetaObject = {d = {superdata = 0x0, 
        stringdata = 0x35d6ba0 "QObject", data = 0x35d6c40, 
        extradata = 0x362a1b8}}, d_ptr = 0x9c61498, 
    static staticQtMetaObject = {d = {superdata = 0x0, 
        stringdata = 0x35e0380 "Qt", data = 0x35e3700, extradata = 0x0}}}, 
  static staticMetaObject = {d = {superdata = 0x80c0540, 
      stringdata = 0x35e87a0 "QEventLoop", data = 0x35e87c0, 
      extradata = 0x0}}}
	returnCode = -1
#26 0x011a5dd7 in QApplication::exec () at kernel/qapplication.cpp:3525
No locals.
#27 0x0807a765 in main (argc=6, argv=0xbf822334)
    at ../../../../apps/dolphin/src/main.cpp:94
	about = {d = 0x9bafd70}
	options = {d = 0x9bc33d8}
	app = {<KUniqueApplication> = {<KApplication> = {<QApplication> = {<QCoreApplication> = {<QObject> = {_vptr.QObject = 0x80a9108, 
            static staticMetaObject = {d = {superdata = 0x0, 
                stringdata = 0x35d6ba0 "QObject", data = 0x35d6c40, 
                extradata = 0x362a1b8}}, d_ptr = 0x9bd9518, 
            static staticQtMetaObject = {d = {superdata = 0x0, 
                stringdata = 0x35e0380 "Qt", data = 0x35e3700, 
                extradata = 0x0}}}, static staticMetaObject = {d = {
              superdata = 0x80c0540, 
              stringdata = 0x35e8820 "QCoreApplication", data = 0x35e88c0, 
              extradata = 0x0}}, static self = 0xbf82215c}, 
        static staticMetaObject = {d = {superdata = 0x362e7e4, 
            stringdata = 0x183e860 "QApplication", data = 0x183ea60, 
            extradata = 0x0}}}, static staticMetaObject = {d = {
          superdata = 0x1a07f48, stringdata = 0xc56500 "KApplication", 
          data = 0xc566a0, extradata = 0x0}}, static loadedByKdeinit = false, 
      static KApp = 0xbf82215c, d = 0x9c26060}, static staticMetaObject = {
      d = {superdata = 0xc7eba0, stringdata = 0xc56dc0 "KUniqueApplication", 
        data = 0xc56ec0, extradata = 0x0}}, d = 0x9c85c50}, 
  static staticMetaObject = {d = {superdata = 0x80c0820, 
      stringdata = 0x80a8fe0 "DolphinApplication", data = 0x80a9080, 
      extradata = 0x0}}, m_mainWindows = {{p = {static shared_null = {ref = {
            _q_value = 14725}, alloc = 0, begin = 0, end = 0, sharable = 1, 
          array = {0x0}}, d = 0x9ddad90}, d = 0x9ddad90}}, m_lastId = 1}
	__PRETTY_FUNCTION__ = "int main(int, char**)"

[Comment from a bug triager]
I just checked this bug myself again (using KDE SC 4.5.3), with the steps on comment 0; and I could still reproduce the crash with the following backtrace:

#8  0xb71c3181 in KJob::kill (this=0x8a784c8, verbosity=KJob::EmitResult)
    at /home/dario/ArchPKG/abs/kdelibs/src/kdelibs-4.5.3/kdecore/jobs/kjob.cpp:106
No locals.
#9  0xb75f2095 in KUiServerJobTracker::Private::_k_killJob (this=0x8ba4460)
    at /home/dario/ArchPKG/abs/kdelibs/src/kdelibs-4.5.3/kdeui/jobs/kuiserverjobtracker.cpp:59
        job = <value optimized out>
        jobView = 0x8a949a0
#10 0xb75f2101 in KUiServerJobTracker::qt_metacall (this=0x8b07cc0, 
    _c=QMetaObject::InvokeMetaMethod, _id=<value optimized out>, _a=0xbfcc091c)
    at /home/dario/ArchPKG/abs/kdelibs/src/build/kdeui/kuiserverjobtracker.moc:100
No locals.
#11 0xb6f7fc4a in QMetaObject::metacall(QObject*, QMetaObject::Call, int, void**) () from /usr/lib/libQtCore.so.4
No symbol table info available.
#12 0xb6f8fa9f in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) () from /usr/lib/libQtCore.so.4
No symbol table info available.
#13 0xb77602b7 in OrgKdeJobViewV2Interface::cancelRequested (this=0x8a949a0)
    at /home/dario/ArchPKG/abs/kdelibs/src/build/kdeui/jobviewifacev2.moc:122
No locals.
#14 0xb77604bb in OrgKdeJobViewV2Interface::qt_metacall (this=0x8a949a0, 
    _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0xbfcc0a24)
    at /home/dario/ArchPKG/abs/kdelibs/src/build/kdeui/jobviewifacev2.moc:98
No locals.

I have quited KDE for quite a while and now i'm back.
Now i'm using KDE SC 4.6.0 on Arch Linux
i tried to repeat the steps in the report
and the bug seems to have been fixed, now it shows the file is already exists in dolphin status  when i do the "undo" step.
So i think this bug can be closed. can someone else verify?

Seems to be fixed.

Operating System: Manjaro Linux 
KDE Plasma Version: 5.17.4
KDE Frameworks Version: 5.66.0
Qt Version: 5.14.0