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

[Adam Davis]

On Tuesday, 26 April 2016 17:01:41 UTC+1, Adam Davis  wrote:
> 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!

Here's my section of code:

with open(class_code+".csv", 'a') as csvfile:
        fieldnames = ["Name", "Score 1", "Score 2", "Score 3", "Average"]
        writer = csv.DictWriter(csvfile, fieldnames=fieldnames)
        #writer.writeheader()
        writer.writerow({"Name": name, "Score 1": score1, "Score 2": score2, "Score 3": score3})

It prints when requested as:

Adam,1,0,0,0.3333333333333333
Jake,9,10,6,8.333333333333334
Tom,5,3,3,3.6666666666666665

So basically, if anyone called Adam, Jake or Tom tries the quiz again, it will not let them.
Thanks