Do I need to amend my tax return if I already filed my federal return with the d40-b form but I actually don’t need to file it?

If your state returns were completed in error, then they will need to be amended to adjust them. However, check the rules to see if you do need to include the form.

To pay your taxes due to another state in your return, that state may need to be amended if it does not currently show a tax balance due.

Form D-40B is a Non-Resident Request for Refund from Washington DC. Per their instructions, Who must file a Form D-40B:

Any non-resident of DC claiming a refund of DC income tax withheld or paid by estimated tax payments must file a D-40B. A non-resident is anyone whose permanent home was outside DC during all of 2020 and who did not live in DC for a total of 183 days or more during 2020.

For more details, see: I need to amend my state return