Our IRS Account Transcript indicates two Tax relief credits AND two refunds of 1200 and 2400. Why did IRS later withdraw $1200 from our checking account?

Only the IRS can answer that, as they do not notify TT.  However, it sounds like they denied a Recovery Rebate Credit for $1200, perhaps because their records indicated you were already sent it.  Wait for a letter from the IRS.

IRS letters explain why some 2020 Recovery Rebate Credits are different than expected | Internal Revenue Service

I am happy to have gotten a reply that seems informative, but I can't really evaluate it until I hear from the IRS (if ever :-)).

It sounds like you claimed a credit when you actually received a payment from the IRS. If you filed jointly with no children, in 2020 you should have received $2400 in the first payment and $1200 in the second. That is the full amount that you can expect to receive in 2020.

If you received claimed additional credits, the IRS recognized that and withdrew the excess amount. That would be my summation.