This is in validation code:
=IF(COUNTIF(A:A,A6)=1,TRUE,INDEX(L:L,MATCH(A6,A:A, 0))<"")

Okay its working, but its not working Please See the following examples.

Scanario 1:
Col A Col L
Account Number Deactive
023-64000 Deactive

Result: I enter the same account number it lets me. This is correct.

Scanario 2:
Col A Col L
Account Number Deactive
023-64000 *NOTHING ENTERD*

Result: I enter the same account number it will not let me. This is correct.

Scanario 3:
Col A Col L
Account Number Deactive
023-64000 Deactive
023-64000 *NOTHING ENTERD*

Result: I enter the same account number it lets me enter the account number
again This is incorrect. Our validation only checks 1 instance to see if L is
populated. It needs to check all instances of duplicate account numbers if L
is empty in any of the instances it will not let me enter the account number
again?