A PHP Error was encountered

Help me plz
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A PHP Error was encountered

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at /home3/ab46993/public_html/app/controllers/user.php:1)

Filename: libraries/Session.php

Line Number: 689
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Lots of things can cause that. The main ones are:

An error occurred and output a message to the screen before the headers were sent
You have whitespace before your opening <?php or after the last ?> tags