I want to create a login page and when i click submit button it will store in database,but the problem is that i want to store only distinct username..anybody please help me.

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C#

ASP.NET

Actually i already created a login page,i.e.username and password and also connected to database,but when i click submit button it stored in database,but when i gave same username it also stored in database ,i want only distinct username,can anybody have some idea??

thanks in advance.