After the WI has gone through, go into the file and remove the WI check box to efile WI. Then open WI and save and close.

Then open the MFS for the one not yet e-filed. Save and close. Then it should be okay to go.; it shouldn't be  duplicate SSN any more.

After it is done, you can go back into the WI and check the box again, open WI, save and close.