My efile return has been rejected twice because it says my spouse's SSN could not be used as a primary SSN on another return. How can I fix this?

you can't. when the same SSN is used on more than one return, the IRS will not allow e-filing because it doesn't know which claim is proper.    you'll have to file by mail.    eventually, it will get around to both claimants asking for proof  they're the one entitled to claim 

Are you filing Joint?  Did she file her own return?  Or sign up on the non-Filer Page  to get the Stimulus Check?  Only thing to do is double check the ssn or mail your return.