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