On a two way sync two records can both match one record on the other side, for example two HubSpot contacts with the email of one GoHighLevel contact.
The record that already syncs keeps syncing and the second one waits. Nothing is merged and the sync carries on.
You get a notification, and the integration's Conflicts tab shows both records. Use the other one instead or keep the current one, one at a time or many at once. A history keeps every resolved conflict with who resolved it and when, and a choice can be switched back or reopened.