Check the «System ID» column in the invoice table. The column may be hidden by default — add it via the column selector. It shows the ID the invoice has in the accounting system.
If two invoices have different System IDs but the same amount and content, it is probably a genuine duplicate registration in the accounting system — not an error in AIMZ. You can then safely reject one of them via the ⋮ menu (three dots) to the left of the invoice. Keep the one that came in last, as it is often the corrected version.
Read more about the Rejected status in Invoice linking.