TeamRaiser shortcuts have to be unique across your entire site. So if two people try to use the same shortcut in any event ever, they will get that error. To allow shortcuts associated with out of date events to be available for reuse, follow these steps:
  1. Unpublish the TeamRaiser event.
  2. Archive the TeamRaiser event.
  3. When viewing the list of events, check the check box at the bottom of the page that says "Show Archived TeamRaisers in List".
  4. Find the archived event and click on "Release Shortcut URLs" to release the shortcut URLs for reuse.