Duplicated posting of sales invoice

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Hello

We have a duplicated SI with the exact same document ref, posted to our ledgers and I don't understand how it's happened. As the transaction numbers are only 12 apart, and the audit trail shows them as being posted by the same user, it looks like it could have happened when the sales invoice was posted, as they are within a batch of similar transactions.

To note in case it's relevant: We use auto-numbering for SI doc ref. We create customer quotations, mark as won, and then post to ledgers.

I can obviously resolve the issue by removing the offending transaction, but any ideas on what happened? and if we can prevent it happening again.....?