I think this raises from the fact that they are the same id but different damage values.
RP2pr2 Marble is 122:0 and Basalt Cobble is 122:3 so I guess it just doesn't pay attention to the damage values.
Nice catch. It was mentioned in the additional pipes thread as well, was that you? Anyway good thing to not ignore metadata
Fixed for the next release, not worth a hotfix though.