Eddie McClintock, Stana Katic Eddie McClintock, Stana Katic

As promos for Monday's much-anticipated Castle wedding episode have already suggested, there's one major final obstacle standing in the way of Castle and Beckett's happily ever after: Beckett's already married!

In the exclusive sneak peak from Castle's Season 6 finale below, Beckett (Stana Katic) pays her "husband," Rogan O'Leary (guest starEddie McClintock), a visit at the dive where he works as a bartender. And although Rogan (who clearly has some problems with the ladies) is excited to see "Kit Kat," he is as surprised as the rest of us to learn that he and Beckett tied the knot during a drunken trip to Vegas 15 years ago.

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Although Rogan initially hopes to give married life a try ("Heck, we have been married this long — maybe we should just give it a chance!" he says), he ultimately agrees to sign a dissolution of marriage agreement — but only if Beckett helps him do something first.What do you think his request is? Can you believe Beckett has been married all this time? And do you think she and Castle ( Nathan Fillion) will make it down the aisle? Watch the sneak peek below and don't hold your peace in the comments below!

Castle's Season 6 finale airs Monday at 10/9c. Watch the season's penultimate episode here.