I don't know if this is an addon or if it's part of the default UI. When I hover over MOST of the motifs that I know, it says "Already in your library." If I do not know the motif, it says "Use to add to your lore library." This is in the tooltip. Is this part of the default UI or is it being added by an addon? Either way, it's apparently bugged for mercenary. I just bought three mercenary pages because the tooltip indicated that I didn't know the pages. Turns out that I already knew them and wasted gold on something that I didn't need. That's aggravating. If it's part of the UI, it needs to be fixed. If it's from an addon, I need to figure out which one so I can post a bug report at esoui. Thanks!
Example:
http://i.imgur.com/HoIEl4q.png