Drop-down list no repeats

Hi,

I have a drop-down list (Data Validation > Allow List) for a number of cells, 12 I think, across a singe row. The values for the drop-down menu are in another, hidden, worksheet.

I would like it so the user can select any option in the drop-down list once and only once. For example, if they select Project1 in column A they then shouldn't be able to select Project1 in column B. Any ideas how I can achieve this? Ideally I would prefer not to use macros, but if needs must...

Many thanks