Conditionally formatting cells if their value equals any value of another column

Question

I have data in the A and B columns. B column\'s data is mostly duplicates of A\'s data, but not always. For example:

Answer 1

Here is the formula

create a new rule in conditional formating based on a formula. Use the following formula and apply it to $A:$A

=NOT(ISERROR(MATCH(A1,$B$1:$B$1000,0)))

here is the example sheet to download if you encounter problems

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UPDATE
here is @pnuts's suggestion which works perfect as well:

=MATCH(A1,B:B,0)>0