From Molly Reynolds at Brookings, from her home county in PA:

I voted by absentee ballot (or received a ballot but did not use it). May I vote in person?
Yes. In fact, the Election Code provides that unless you are 65 years of age or older, you must vote in person if possible. At the polling place, the Judge of Election will note on the appropriate form that you appeared to vote in person, and you will sign in and vote as if you had not requested and submitted an absentee ballot. When the absentee ballots are counted at the Board of Elections office after the polls close, your unopened absentee ballot will be marked VOID and will not be counted. Should you encounter any difficulty at the polls on election day, urge the official to contact the Office of Election Services at 610-559-3055. Note: Voters age 65 or older are permitted by law to submit an “alternative ballot” if their polling place is not handicapped accessible. These voters may also go to the polls to cast their ballots.