Help with look up function

I have this situation:
A1 CLIENT 1
A2 CLIENT 2
A3 CLIENT 3
A4 CLIENT 4
A5 CLIENT 5
A6 CLIENT 6
A7 CLIENT 7
in column C, I have zip codes for clients (some zip codes are simillar)

C1 10012 E1 A
C2 12133 E2 B
C3 13323 E3 C
C4 10012 E4 A
C5 13323 E5 C
C6 19989 E6 B
C7 13323 E7 C

(I alligned zip codes like that to be easier to understand)
in column E, I need a formula that will assign a different letter (or number) for clients that have the same zip code and for those that have different zip code another letter; example
E1 A
E2 B
E3 C
E4 A
E5 C
E6 B
E7 C

Thank you in advance.