Use Data validation on C1:c20 with a formula of

=NOT(SUMPRODUCT(--($A$1:$A$20=A1),--($C$1:$C$20=C1))1)

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HTH

RP
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"Jay" wrote in message
...
I have a column with 20 cells (a1:a20). Those cells can only be filled
with
5 different text values. In another column (c1:c20) I want to force a
unique
value being entered base on the text in a1:a20. For example:

Bob 1
Sally 2
Ted 1
Ted 3
Sally 1
Joe 1

Invalid would be Sally assigning a value of 1 to two of her entries:

Bob 1
Sally 1
Ted 1
Ted 3
Sally 1
Joe 1