Welcome to the Community. Let me assist you with your concern about the journal numbering in QuickBooks Online.
Currently, a warning notification for duplicate journal entries is unavailable. As a workaround, you can run the Journal report to view the list of transactions, as well as the transaction numbers.
For additional reference, you can use the following articles to reverse or remove a journal entry in QuickBooks: Reverse or delete a journal entry.
Please know that I'm just a post away if you have any other questions. Take care.