compare textboxes

I have  4 different textboxes for emails(CC)

If user enters an email address in textbox1 i dont want him to add the same email address in the rest of the 3 textboxes (similarly for others too).

 I can do can write if and else statments for comparing these textboxes or do string.compare

other than using if and else for all the 4 textboxes is their another way in asp.net 2.0 where i can compare the values? and if they are same display it to ths user?
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