Trying to avoid repetition in database

[Ozz]

I use a form to send links to my site.
Its a link log type site.
I want to inform submitter when a link exist on database to avoid repetition.
table name is weblog column name is body in my MySql database.
How can I do that in php with a database query ?
Thanks

[PyroX]

$res=mysql_query("SELECT BODY FROM weblog WHERE body='$linktotest'",$db);

if(mysql_num_rows($res)<1){

GO AHEAD AND INSERT

} else {

LINK EXISTS

}