Merging a user's object relationships to another user, with a condition, in mysql

Question

this is my first ever S.O. question :)

I am trying to "merge" a temp user to an existing user, and all their associated object relationships - one being "post_relationships"

The problem is.. if both users have a relationship to the same post, a simple INSERT INTO ... SELECT statement yields a DUPLICATE KEY error..

ALSO, if one of the users has a higher permission_level, i'd like to keep the HIGHEST permission_level of the two users.

I have constructed a query that works, but I am wondering if there is a better way to accomplish this. My fear is that as the table grows, this query won't be as efficient.

INSERT INTO post_relationships
            (id_user, id_post, access, label, time_created)
        SELECT 
            old.id_user,
            new.id_post, 
            new.access, 
            new.label, 
            new.time_created 
        FROM 
            post_relationships AS old,
            post_relationships AS new
        WHERE 
            old.id_user = '$id_old' AND 
            new.id_user = '$id_new' AND
            new.id_post = old.id_post AND 
            new.access > old.access
        ON DUPLICATE KEY UPDATE
            access= new.access,
            label = new.label

It always forces an UPDATE when the condition

new.access > old.access

And this has me worried that there's some overkill code here. Any suggestions?

Thanks!