This sounds like a classic dunning letter. Look carefully...there should be a blurb in there somewhere about having 30 days to notify them (in writing) that you dispute the debt in question.
I would send them a DV letter right away (certified mail, return receipt).
If you don't challenge it, and the debt in question is less than 7 years old, they could put it on your CR. Not something you want showing up in the midst of trying to buy your 1st home.
Good luck! 
Starting Score: EX: 661, EQ: 662, TU: 694 [Lender Pull 12/30/2010]
+1 You need to get that DV in the mail. Being as you have just received a dunning letter, it will be considered timely and thus cease collection activity until they validate that the debt is yours. You say that you have never had a citi account, if this is the case then perhaps it's a case of mistaken identity or identity theft. Get that DV in the mail so that you know where you stand.
Starting Score: 504
Current Score: EQ FICO 819, TU08 778, EX "Who Knows?" Quizzle 756
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LVNV is just the name of the CA. They aren't an OC of any type. It's just the name of the company.
Starting Score: 504Current Score: EQ FICO 819, TU08 778, EX "Who Knows?" Quizzle 756
Goal Score: All Scores 760+, Newest goal 800+
Take the myFICO Fitness Challenge
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