Trying to create a data validation field that doesn't allow blanks or duplicates

Hi,
i have a table for gym members and each member is assigned a unique member number.

spreadsheet.PNG

I wish to create a data validation field that wont allow you to leave the cells blank or use the same code twice.

this was my attempt that failed: =AND(ISERROR(MATCH(A:A, A2, 0)) <>FALSE, A2<>""

Any help would be greatly appreciated, thankyou in advance

For A2 try this

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