PA and MD have a reciprocal agreement such that PA agrees not to tax the W2 earnings of a MD resident (and vice versa). Thus no PA taxes should have been withheld, only MD taxes.
You need to file a MD resident return reporting ALL income earned, regardless of where. You would only file a PA return if you had self-employment, business, or rental income that originated in PA.
Just keep the extra W2 for your records.
