diff --git a/pkg/services/sqlstore/migrations/ualert/ualert.go b/pkg/services/sqlstore/migrations/ualert/ualert.go
index 03a4ee47834..ecef514ef16 100644
--- a/pkg/services/sqlstore/migrations/ualert/ualert.go
+++ b/pkg/services/sqlstore/migrations/ualert/ualert.go
@@ -459,6 +459,9 @@ func (m *migration) writeAlertmanagerConfig(orgID int64, amConfig *PostableUserC
 		return err
 	}
 
+	// remove an existing configuration, which could have been left during switching back to legacy alerting
+	_, _ = m.sess.Delete(AlertConfiguration{OrgID: orgID})
+
 	// We don't need to apply the configuration, given the multi org alertmanager will do an initial sync before the server is ready.
 	_, err = m.sess.Insert(AlertConfiguration{
 		AlertmanagerConfiguration: string(rawAmConfig),