Breaking Graves and Memorials (all types from what i've seen) give you 2 copies of the gravestone.

Gravestones 1.0.6
Extended 1.0.5

I know this has been closed, but I (finally) figured out the root of the problem.

It appears to be cause by Multi Mine by AtomicStryker ( https://minecraft.curseforge.com/projects/multi-mine ).

Why it happens I don't know, but removing the mod immediately fixes the problem, for skulls and graves, so sorry for disturbing you.

Also just noticed, but not sure if this info is helpful, that this also seems to affect all skulls and heads as well.