Created attachment 111207 [details]
Duplicate thumbnail of 1st Dolphin window
Sometimes there is a duplicate thumbnail of the first window in a group of windows. Unfortunately I have not found a way to reliably reproduce the bug, nor do I have any idea what causes it.
I have found the bug on openSUSE Tumbleweed (Plasma 5.11.95 to 5.12.2 and KDE Neon (only tested 5.12.1).
Killing plasmashell and restarting it seems to make the problem go away.
*** This bug has been marked as a duplicate of bug 389469 ***
Thanks for your report. We're aware of and currently working to resolve this bug. Sorry!