You can do it either way.
Since the taxes are already paid, you could hang on to the money and you'll obviously have a surplus when you do the next analysis. Thus, you would handle the surplus according to RESPA requirements. OR
You could contact the customer and say "I noticed you paid your taxes. Do you remember we have an escrow account account set up to pay these for you (and your insurance . . . )? If you'd like, I can cut you a check (OR when your analysis comes due next in _____, we'll be cutting you a check) for X amount. What would you like to do.
To be clear: You don't have to contact the customer and you don't have to cut them a check at this time