If you think "duplicate" (or "unique"), think about using a hash!

foreach $obj (in $ou){
 ...
 print "$obj->{givenName}\n" ; 
 ...
}
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In that loop, just create a hash-entry for each user, using $obj->{givenName} as the key...

... unless, however, an entry for that key already exists. In this case you have found a duplicate :-)

HTH, Rata