Duplicate Art on Wall

Hi, how do I easily remove duplicate art for my movies? Many, though not all movies have duplicate art shown (I'm using BigWall list option). When I click the first (usually), it says '... no longer exists, would you like to remove?'. They're too many to delete one by one.

I'm using SPMC 16.4.2 and FW009, and not sure what caused it so I just created a thread.