I can't reproduce on created wiki, so there you won't have duplication.. Such user appears twice and only on main (default) wiki, {{userPicker}} loads suggestions [here|https://github.com/xwiki/xwiki-platform/blob/ce7df6305872d966fd5bdcb1ad11a6ecf49013d0/xwiki-platform-core/xwiki-platform-web/xwiki-platform-web-war/src/main/webapp/resources/uicomponents/suggest/suggestUsersAndGroups.js#L81-L83.]. because the wiki user scope is {{LOCAL_AND_GLOBAL}} both queries hit the same wiki, so every user (including the current owner) appears twice. https://github.com/xwiki/xwiki-platform/blob/ce7df6305872d966fd5bdcb1ad11a6ecf49013d0/xwiki-platform-core/xwiki-platform-rest/xwiki-platform-rest-server/src/main/java/org/xwiki/rest/internal/resources/classes/UsersClassPropertyValuesProvider.java#L97-L98
XWiki groups already contains [ fix for this. |https://github.com/xwiki/xwiki-platform/blob/ce7df6305872d966fd5bdcb1ad11a6ecf49013d0/xwiki-platform-core/xwiki-platform-rest/xwiki-platform-rest-server/src/main/java/org/xwiki/rest/internal/resources/classes/GroupsClassPropertyValuesProvider.java#L83-L90] for this.
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