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Put myself in an awkward situation.

Accepted an offer yesterday and withdrew from consideration from a number of firms, but forgot to withdraw from Firm X. Firm X just called and left a voicemail asking me to call back, presumably to extend an offer.

Should I tell the hiring partner on the phone that I've accepted another offer (with a firm in the same market), or should I just say thanks and follow up with an email this weekend/next week saying I've accepted an offer elsewhere?

What is the least awkward/best way to maintain relationships here?

You can just call back and let them know you just accepted an offer. You do not need to specify when you accepted the offer unless they specifically ask. I had a friend who accepted a clerkship with a judge and a few minutes later another judge on the same court called to extend an offer, and she had to tell the second judge she had just accepted. It's not as big a deal as you think and it's better to let them know up front so they can move onto the next candidate.