Primary key and unique key in django

I had a custom primary key that need to be set up on a particular data in a model.

This was not enough, as an attempt to insert a duplicate number succeeded. So now when i replace primary_key=True to unique=True it works properly and rejects duplicate numbers!!. But according this document (which uses fields).

primary_key=True implies null=False and unique=True.

Which makes me confused as in why does it accept the value in the first place with having an inbuilt unique=True ?

Thank you.

Updated statement:

   personName = models.CharField(primary_key=True,max_length=20)

Copyright Notice：Content Author:「Nagaraj Tantri」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/6039443/primary-key-and-unique-key-in-django