@Osalo Yes, when attending a party or just viewing another player’s house shows your heirlooms as duplicates if there are heirlooms on that player’s house.
This issue has been going on for a year at least now (since the neighborhood update?) and I’m sad to see that the studio has not resolved the issue or even made it so that players cannot sell their heirlooms (and stop offering same heirlooms in the store or the heirloom boxes). Or acknowledged the issue and communicated to players that do not sell the duplicates unless you are 100% sure that they actually are duplicates.
Losing (rare) items so easily in this game is a major issue.