Create a drop down list using data validation that prevents duplicates being added

I have a spreadsheet with a name field in the B Column (B2:B49). These names are added using a drop down list, created using Data Validation.
I'd like to prevent duplicates being added into this column when I use the drop down option. Eg: at the moment I can select 'John' in B2, and also 'John' in B3. I want Excel to prevent me selecting 'John' in B3 because I have already selected it in B2.

A search of the internet leads me to believe this is impossible - any suggestions?

Thanks