Validating Unique Email And Username?

I am writing a registration form which will store all the information to the database. I need to validate some data before saving like empty and isunique.

I have username and email address field in my form. I used CustomValidator to call a function check unique to execute my code to check the given email address with the list in database.

It works fine for new case but when i am for edit case it does check with itself and reports already in use.

My Steps:

1.) Page load

If edit case retrieve data and populate them in control like. txtemail.text = dbrec("email")

2.) submit action

if page.isvalid() then

end if