If we're talking both item card and natural drop, yeah, it has an extremely low probability but it could happen that both chances would kick in and you'd get 2 OBBs off the same monster. I don't really feel like bringing out the math for this, just expect this to never happen. The good news is that you get 2 chances per said monster. Think of it like how some monsters will drop the same item multiple times normally and go from there.
If we're talking two of the same item card in your equipment though, they'll stack up I believe as per card rules. So only one chance, but the chance will be doubled. This doesn't apply to cards that have been limited, like how I hear only one Blazer card works.
(Though really it's the same theoretical chance whether the cards stack up or not or whether they roll the chance twice. The only difference is that once in awhile the chance of both dropping would intersect and you'd be rewarded with two, but the chance again is pretty much a zero.)