Prevent adding a user in column A if it exists in column B

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Hi all,

I have a list which as two people or group column, 1) Sign off - Awaiting and 2) Sign off - Completed.

Both columns accepts multiple users.

 

I want to prevent adding a user in Awaiting column if the user exists in Completed column.

 

How can I achieve it?

Many thanks in advance.

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