Due to bug JRASERVER-64998: Prevent the same custom field from being added to multiple screen tabs in JIRA's Java API, by some custom add-ons, the same field gets defined at the same screen multiple times:

This causes REST API issue and other cosmetic problems in Jira interface. It would be good if an database constraint could be added in the fieldscreenlayoutitem table by default for combination of field fieldidentifier and fieldscreentab, so this wouldn't be possible anymore.