I'll try interpreting what I think you mean...
I want to prevent duplicate entry against any account
Suppose i have an account (4422A) and i want it to pay only one in a month
If i enter this account twice access don't allow this entry.
You can easily prevent duplicate entries by indexing the account field and specifying no duplicates
However, I think you mean that it would be OK to pay that account again the following month.
So, you could use a field called YearMonth or similar where this month would be 2201 & Feb 2022 would be 2202
Now you could validate entries using a DCount expression. Something like:
- If DCount("Payment", "tblPayments", "AccountID = " & Me.AccountID And YearMonth = Format(Date,"yymm"))=0 Then
-
'allow payment
-
End If
This undoubtedly needs modifying but it might get you started
EDIT Crossposted on at least two other forums