how to reproduce? i have only one

I can reproduce this reliably with VLC media player. Launch VLC for some file, quickly close it and launch it again. Each time you do this you will get another MPRIS plasmoid.

Do you have "one instance only" enabled in vlc? Their mpris support is quite...unusual.

Git commit 58490813dc775fc78934b6948a7f5a12efd2d3f6 by Marco Martin.
Committed on 13/05/2014 at 12:19.
Pushed by mart into branch 'master'.

slightly defer plasmoid init

since when the plasmoid is gettng created other events may be processed,
slightly defer it's init() so that if a malformed mpris implementation
(hello VLC) adds the plasmoid to tasks immediately, so we can ensure
that every plasmoid gets onle and only one instance

M  +1    -1    applets/systemtray/plugin/protocols/plasmoid/plasmoidtask.cpp

http://commits.kde.org/plasma-workspace/58490813dc775fc78934b6948a7f5a12efd2d3f6

it was reproducible with vlc.
now it should try harder to check if there are no duplicates in the systray