how can i stop the same thing being repeated in a column?



I have a list of contract numbers at the end of each contract number (whoch
is 6 digits long) I have an a1 or a2 or a3...etc. The a stands for
application and the number represents the application number.
I want to format the cells so that no application number is entered in twice.
ie. if for example 022078a2 had already been entered. But someone tries to
re-enter it again an error message will appear.
I have already have an error message appearing if the someone enters data in
to that colunm that doesnt match the following format 'nnnnnnan'


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