It's possible that this is a scam, but bear in mind that the IRS does occasionally send letters out erroneously; they even have procedures for erroneously-issued 4883c letters. And, you're right, it is possible that someone submitted a fraudulent return using your information after you received your refund -- it can take some time for information to post to the IRS' Master File.
The IRS contact phone number on the 4883c letter should be (800) 830-5084. If that's the number you're seeing, the inquiry is legitimate. Even though you've received your refund, you might go ahead an call, if you have time, just to check what's up, and to alleviate any worries that you might be having about the letter.
For more information, please see the TurboTax FAQ below.
I got a 4883C letter from the IRS
