If you have an external gift on your registry that is getting purchased multiple times, or is frequently alternating from purchased to available, it's likely not updated as purchased on the external registry the gift is being synced from.
If an item is marked as available on the external registry, Zola will sync to match that external registry. This item isn't showing as purchased on Zola because we can only pull information in, we can't overwrite their system. The best way to avoid duplicate gifts is to mark gifts as purchased on your external registries as soon as they start showing as purchased through Zola.
If you'd prefer to skip that process, you can always help import the individual gifts as cash so that you don't have to manage in two places!