Hi,
Strictly speaking, only one UAN should exist per member. They have now streamlined the process so that if a member already has one UAN, the employer will not be able to allot a new UAN number. Hopefully, your case should not have happened in the past.
Now, if you have two active UANs, you should deactivate the previous UAN and transfer the EPF account to the new one. You can transfer the old balance from the previous UAN to the new UAN.