If you put a Item Frame on a block that loads a comparator and this comparator points north or east, while this Item frame are in a Chunk and comparator and block in another, when loading the game or load those Chunks, the game multiplies entities in the Item Frame, the Item Frame ifself and also the entities there are in the Chunk.

This video in spanish shows the bug: http://youtu.be/JbwBoU8oWP0
Minute 8:00 show the duplication: http://youtu.be/JbwBoU8oWP0?t=8m