This is something that USCIS may be able to help you with--try calling the 800 number, and confirm with the person you speak to that their system reflects that you already had your biometrics taken. You may just be getting duplicates because of a glitch in their system, or there may be a legitimate reason for the second appointment.
Sometimes the 10 fingerprints done at the first biometrics appointment are unreadable and unusable for the FBI check. When this happens, the applicant is scheduled for a second biometrics appointment. I agree that you should try to resolve this via the 800-number and hopefully an escalation to a USCIS officer, but even though it may be inconvenient it may be wise to attend the second biometrics appointment too, because it will be even more inconvenient if your I-485 is denied.