Multiplayer removing item from frame duplicates it

Also in 1.01 (not offered from dropdown).
I have marked this private for obvious 'cheating' reasons. I hope that's appropriate (the pop-up window gives no useful advice on this).

To reproduce: HostPlayer sets up survivor multiplayer game. MultiPlayer1 joins. Either player places an item (any, it seems) in an item frame. MultiPlayer1 removes it, and two drop out. It's slow-but-infinite resources.

I've reproduced this many times with a potion, a block (diamond), a flower and a few others and it's completely reliable.

It also happened on single player once when near a mob spawner with a lot (tens) of mobs trapped and the game had become laggy.

PS it's very hard to search for 'duplicate' in the description as it seems to search for duplicate reports (and you get many hits...!)