This problem has been going on for weeks now, and at last U.S. Citizenship and Immigration Services (USCIS) has issued an official statement acknowledging it: The agency has been issuing duplicate biometrics appointment notices.
In other words, the same person might get an appointment notice for two different dates to visit an Application Support Center (ASC) -- especially confusing for people who have already attended one biometrics appointment (at which a photo and fingerprints are taken in connection with an application for an immigration benefit -- see "What to Expect at a USCIS Biometrics Appointment" for details.)
Unfortunately, the fact that USCIS may have made a mistake doesn't let you off the hook -- you may have to confirm the error and figure out the best way to deal with it. To that end, USCIS suggests: