While registering the user with django form, it gives "UNIQUE constraint failed: auth_user.username" error

When I try to register a user using Django form, it gives me "UNIQUE constraint failed: auth_user.username". When I tried registering the first user, it worked, but I couldn't see that entry in my database. When I tried registering the second user, it's giving me this error. Please, help me

Saving data from HTML form into the database

I want to save the data entered by the user into the HTML form(pure HTML) into the database. I'm a complete newbie in Django, what I found is Django forms can do the job, but still I can't figure it out. Please, provide me with a detailed answer/link for this. I'm using Django 1.9. Also, I wish