Once the IRS computers have an SSN marked as "claimed" for the tax year, that's it. You flat out no way, no how can e-file any tax return that has one or more SSN's on it that has already been claimed on another tax return. Period. End of Story. There is no work-a-round.
If after confirming you have all the SSNs for all tax filers and all dependents entered correctly, you still can't e-file for the same reason, then you have no choice but to print, sign and mail your return to the IRS. Again, there is no other option.
When the IRS gets your return and becomes aware that one or more SSNs on your return have been claimed on another return, both you and the other person that claimed them will be audited on that specific item. Loser pays back some of their refund with interest and penalties. So if your claims are legit, nothing to worry about.
Will it delay your refund? YES
How long will it be delayed? No way possible to tell with the IRS being closed due to COVID-19. Could be six weeks, six months, or even longer.
That rejection means that either your own SSN or the SSN of a spouse or dependent on your return has already been entered into the system and you will not be able to e-file. Unfortunately, if you know that the SSN you entered is correct and should be on your tax return, the only solution is to print, sign and mail your return and let the IRS sort it out.