Whenever I have had an account (parent or scout) show us as having an email already in use by another account, it has been because there actually is already an account using that email address.
The “why” of the email address already be in use has generally been:
- The adult is already a registered leader or has an account at my.scouting (e.g. previously registered leader or took YPT even though not a registered leader).
- The email address was associated with someone else (e.g. parents/families sharing an email address) who is already in Scoutbook. Scoutbook requires a unique email address for each account.
- The adult already has a Scoutbook account that they don’t recall having (e.g. their pack used scoutbook, but the parent never logged in).
As to locating the “other account”, it seems like searching based on that email address has always returned the adult I was looking for.
ETA: I hadn’t thought of the possibility that a scout has a parent email address associated with his or her account, as Ed suggested below. That would likely result in no adult being returned when searching based on the email address.