I guess it depends really on how much you treasure your collection, for example even though I have the IDW tpb collections of G.I.Joe and Transformers from back when Marvel was publishing them yet I still have the entire single complete issues that I can't just part with especially with the time and effort it took me to collect them.

When it comes to double, triple or quadrupal dipping yeah I'm guilty of that too, I have like three versions of The Dark Knight Returns and about four versions of Batman Year One.