I'm entering data one row at a time. When I enter a value in column A, I need
to know if that value has been entered (in Column A) before. Column A needs
to be unique values.

What is the best way to verify uniqueness, as I enter a new value?

Should I use a module or can a formula check for?

I'd prefer to have a Warning box pop up indicating "Value already entered in
this column", so I'm assuming a module (VB?) is the way to go.

If you have any suggestions, I'd be happy to read.

Thanks,