Hi, it appears to me, that it is possible to tag a single image multiple times with the same tag if you are fast enough... I noticed when tagging this image with the "bridges" tag:
http://alpha.wallhaven.cc/wallpaper/183068
You can reproduce this issue by typing a tag and klicking on it in the dropdown menu. When you then klick on the [+] Button fast enough, the tag is added a second time.