Okay, I worked in Philadelphia for 3 months in 2020 and lived in NJ. Am In supposed to fill out a tax credit for taxes paid to other states for Philadelphia?

I worked in Philly from January to March.  I then had to work from home in NJ for the rest of the year.  Do I apply a tax credit for the taxes paid to the city of Philadelphia during those 3 months?  I didnt pay NJ state tax during those 3 months.  Is that normal?  Also what is Double-taxed income?