How to compare a field value with each title values?
Finally, I found a solution with the module Field Validation and a PHP Code validation rule associated to my field Synonyms:

$args=array(':type'=&gt;'specimen');
$titles=db_query('SELECT title FROM {node} WHERE type = :type',$args)-&gt;fetchCol();
foreach ($titles AS $title){
if ($this-&gt;value==$title){$this-&gt;set_error();}
}


It works like a charm.