Sales and Use Tax Information
Scroll down to "Frequently asked questions about sales and use tax", then, "Out-of-state purchase FAQ's", then, "What if I pay sales tax to another state?
It reads:
What if I pay sales tax to another state?
Maryland grants a credit for sales tax paid to another state up to the amount of Maryland's six (6%) percent sales and use tax liability.
For example, if you paid a four (4%) percent sales tax to another state, you would be liable only for the difference, or two (2%) percent Maryland sales and use tax when you brought the property into Maryland. If you paid a six (6) or higher percent sales tax to another state, you would not be liable for Maryland sales and use tax when you brought the property into Maryland.
So, yes, you have to pay tax, no you don't have to pay double. If PA sales tax is over 6%, and you pay it, you don't owe Maryland anything. If you pay PA less than 6%, then you owe Maryland the difference between 6% and what you actually paid in tax to PA.