Nope, it could be an epic or a legendary one instead, if we’re speaking about that one item that has duplicate protection.
Among the first few opened packs — absolutely. Especially epics are not that uncommon.
And what are you trying to save them for? When it comes to rare mercs, you get virtually all of them immediately anyway, and it’s the legendaries that require the most packs (I’m talking mostly about the situatuion when a batch of new mercs is released). As for epics, you’ll probably get most of them while looking for a legendary… However, if you have really few packs, you might choose to save them for future epics.
I’ve been mostly assuming that you already have the majority, if not all, of the older mercs. If not, I suppose you might as well keep opening packs.
It takes more packs on average to get all mercs from each newly released batch than you gain from completing the task chains, so it’s never quite sustainable… And keeps getting worse — I’ve written a bit about it, other have noticed it too.
Ultimately, it’s up to you to decide what to do with your packs or gold, but I hope I’ve been able to provide some insight.