This occurs because the user is a member of more than one Content Editor Access Group with access to the content category.

To review the Content Editor Access Groups a user belongs to:
1. Navigate to onMessage > Settings > Pages & Content
2. Mark Display By User
3. Click on the Number of Groups link for the User
4. Note the listed groups
5. Mark Display By Group
6. Review the groups to see which groups are causing content categories to overlap