<?php
$str = 'foo the moo moo cow not the moo cow is the cow the moo cow is the cow not the moo cow bar';
print $str;
print preg_replace('/(\b.*\b)\1/', '\1', $str);
foo the moo moo cow not the moo cow is the cow the moo cow is the cow not the moo cow barfoo the moo cow not the moo cow is the cow not the moo cow bar