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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?

Honestly, I wouldn't stress about it too much. Definitely contact the hiring partner back and when he extends his offer, I'd just let him know that you're really honored and excited to receive the offer, but actually just that same day accepted an offer at a different firm. I wouldn't say thanks then keep them hanging. I think they'd appreciate the heads up since they're trying to fill their class quickly.