Another option would be to have a Worksheet Change Event macro that monitors changes in column A. As soon as a cell in column A gets changed, the macro will copy the value to column B.
Both column A and B have the same data validation setup.
Select a value in column A and the same value will appear in column B (that's the macro at work. There is no formula in column B. The user can select a different value in column B. But when they edit column A again, the current value from column A will be copied to column B again.
This is the code
See attached file for a working example. You need to enable macros. The code is in the Sheet1 module (right-click on Sheet1 and select View Code)
cheers,
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