Hello Antonio,
the GL balanes migration is triggering postings to the corresponding* accounts, so the migration is accumulating the balances. Thus, a migration with inverted amounts will "undo" your first migration. Of course, all postings will still be present, the original ones as well as the inverted ones, so from an auditor's point of view it is recommended to avoid this situation, especially in a production system.
Regards
Michael
(* Please be aware that for subledger accounts the postings are rerouted to migration control accounts 9900xx, as these posted are expected to come via migration of subledger objects like Open Items Receivables etc)