Howw to prevent the duplication of any value in a column within a CSV file (python)

[peakgraphicz at gmail.com]

I am wondering how to make my code function so it does not allow any of the same values to be entered into a column in my CSV file created through python. So I need to read into the CSV file and check if any names have already been entered. If they have, the user must not be allowed to enter this value.

Thanks in advance!