Hello,
I accidentally added the window selector two times in my panel. Normally, you can remove a shortcut by right-clicking it, but it in this case, right-clicking gives you options for the active windows.
Any solution except removing and adding the panel?
Thanks!
Hi,
right click the window selector and make sure it doesn’t say “Lock To Panel”. if it does, take the tick out of the box!. 
Hi Taxicletter,
Right clicking the ‘empty’ panel immediately to the left of your leftmost window button should reveal the options for the window selector, not the individual windows. As Wolfman says, you may need to uncheck ‘Lock to panel’ to make ‘Remove from panel’ active.
This is what I get when I right-click the window-selector:
This is when I click next to it:
(sorry, apparently I can only post one picture per post as newcomer)
I can't see a button to uncheck. My other panel-parts, those I can right-click and slelect to remove...
This is the second menu (right-clicking next to the window selector):
Sorry, that’s not it. What you say works on any other item, but not for the window selector. I now removed the panel and put one back. The Ubuntu menu is different now (no division in “Applications” / “Places” / “System” anymore, that’s in the menu itself), but that’s not a real problem.
Seems like I found some very small but hard to tackle problem 
Thanks for your help! If I can solve it one day, I post it here!
EDIT: I went to system > preferences > display and behavior (or something like that, mine is in Dutch) > MATE tweak > interface and there I selected a different panel-look and then Ubuntu MATE again, which reseted my panel. Again the Applications / Places / System. And I added the window selector again, ONE time now :-).
The window selector has a little mark in front of the first window (:), It may be invisible like mine.
Maybe thats what your after
Look how close I right-click:
The "systeemmonitor" is on the LEFT of the window-selector, so why I get the options for it on the RIGHT of it, I don't know.
Thanks for all the tips, I'm sorry they don't seem to help. The main problem is solved by removing and adding the panel. Anyway, if there IS a solution other then that, I'm still very curious to learn.
Greetings
