Highlight cell if text is a duplicate within the last 31 days

Hi Mike,
Not sure if this what you're looking for but like to help.
Here is my attempt:

=SUMPRODUCT(--($F$1:$F$1000=F1),--(($AC$1:$AC$1000)>=EDATE(AC1,-1))) > 1

I've used EDATE which subtracts by 1 month instead of subtracting 31 days.