I suggest you calculate your AAoA with and without the account, and see which matches your CR.
Its a simple calculation..... age of all TLs in your CR (open and closed) /tot TLs.
I don't see how having an eight year old account would lower AAoA from 25 months to 20 months. If you AAoA is eight years or less, the duplicate account is helping, not hurting. For it to hurt AAoA, your AAoA would have to be nine years or greater.
Your math is incorrect. Age of accounts is determined by the open date, not the number of months reporting. So for the two SunTrust accounts each reports approximately 96 months, not 57 and 1. Assuming 8 years exactly for the SunTrust accounts and that the open accounts are correct, with the duplicate account your average is about 3.9, without 2.9. Since AAoA is rounded down your AAoA is 3 with the duplicate, or 2 without. I would not dispute the duplicate since it is clearly helping.