At most you’ll owe RI $5 for some stupid reason

I’ve worked in MA and lived in RI for almost 20 years, and only had to pay income tax in RI when I worked in RI.

You'll file a return in MA as a non-resident first and then one as a resident in RI. When you do your resident return in RI, you'll put how much tax you paid in MA. You'll need to ask someone in RI what kind of credit RI gives you if it's 100% or not, but usually it is. Example, if you were supposed to pay $6,000 in tax in RI according to your income but you paid $5,000 in MA, then you only owe RI $1,000 (if they give you the full credit). MA does, I earn income out of state, but haven't done it the other way around to know about RI.

You're not really double-taxed. You just pay the highest rate of the two states.

Usually you pay taxes in the state you work in first. Then if your home state has a higher tax rate then the difference is what you owe to your state of residence.

did this for years.

no, you are not double taxed.

A key point that you mentioned, do the non-resident state first. If using tax software then once you start your resident state's return the software will do all the confusing stuff for you. Just answer the software's questions. From my experience doing a friend's some years ago you may owe RI a little bit.

This isn't the procedure for Maine. I just file a Maine State and Fed. Never filed a MA Non res form. If your state taxes your income you are exempt. Been doing it for 13 years. Not once have I ever filed anything with the state of MA, except for unemployment because that's where my employer resides.

I'm not a tax advisor, but if you earn more than $8k in MA as a non-resident, you need to file a return. It could be in your interest to do so anyway as you could very well have overpaid them in withholdings.