How do I calculate what was double taxed if I had an internship in MA? My resident state is DC and I paid rent for the entire year.

[adityacoolkarni3]

I attend college and live in Washington, DC. I was a resident of DC for the 2018 year. I got an internship in Massachusetts over the summer. During the course of the internship (2 months), I changed my address to MA and moved there but still payed rent in DC. I understand I need to file a resident DC tax return and MA non-resident tax return? Am I right in my judgement? 

Another question is, lets say I made $20,000 in MA and only got taxed MA state tax, and I made $20,000 in DC and got taxed in DC. I was not taxed for any of my Massachusetts income in DC. So now when it comes to double taxed income, how do I calculate that?"

[DanielV01]

The amount earned in Massachusetts is the double-taxed income.  It is called double-taxed because both Massachusetts and DC tax you on this income:  MA because you earned the income there, and DC because that is where you lived.  Any amounts you earned in DC is not taxed in Massachusetts.  Prepare your MA nonresident return first so that you can receive a credit for the tax you pay on that income on your DC resident return.