Maybe try putting something like this into your loop

 
unless ($seen{$comment_author2}){
    $seen{$comment_author1} = $comment_author2;
} else { 
    delete $seen{$comment_author2}; 
}
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at the end the hash should only contain unique pairs.

Update

You can put any data into the hash, something like =[$comment_author1,$interactions] would keep more informations if needed.