Everyone with the duplicating diplomas issue, please click "Me Too" here so it gets more attention: https://answers.ea.com/t5/Bug-Reports/NEEDS-INPUT-DU-Household-Inventory-if-filled-by-other-sims/m-p/8458178#M111868
This seems to affect rotational players the most, since new diplomas appear every time you change households.
To give the original sim a copy of their diploma, place it into the inventory of an active household member and use the 'give gift' interaction on the other sim. (Need the Seasons pack for this to work and may have to build relationship first.)
*Or do what Chazzzy did above and temporarily merge your sim with the other household to retrieve the missing diploma.
You can delete the extra diplomas by drawing or placing a small room, hanging them on the walls of the room, then deleting the room. (Click the room to highlight it, then press Delete on your keyboard.)
