I'm not sure what caused this (see attachment), but I saw this when I returned from the nether. they were exact duplicates of the diamond hoe in the item frame. (same durability)
Related issues
duplicates
Attachments
Comments

[media]

Steps to recreate?

come to think of it, I did relog while I was in the nether. so I guess go through the nether portal, relog, spend some time, come back, see this, wat do.png

If the item frame is right on the chunk border (ie block contained within border, frame in next chunk) this can happen.
See this video for more details: https://www.youtube.com/watch?v=u0ejy0IEKXs