The subId collision check in Update ran on every save, unlike the email
check above it. Because Update defaults an omitted subId to the stored
one, any client already sharing a subId was rejected on every later edit
-- even a pure totalGB or expiry change that never mentions subId.
Gate the check on an actual change. Pre-existing duplicates are reachable
because SyncInbound has no such check, and 88a36773 meant to leave them
untouched. Editing a client onto another subscriber's subId is still
rejected, so the typo guard is intact.
a652cb8cea