Go to settings > appearance > Message Tags Create a couple to few tags. Change the background color on any of them and hit okay. Every change to any tag attribute seems to create a duplicate of the most recently created tag.
I should elaborate that if you go to right click a message and tag it; Then you'll see the duplicates in the context menu. The duplicates do not show up immediately in Appearance until you close and re-open kmail.
More details: I think it might be limited to changing the background color. I've tried changing some other attributes and it created no duplicates.