I would say that only the one you received second could be the duplicate. The first one you received is the original and the second might be a duplicate. However, it's only a duplicate if it is exactly the same as the original. Surely there is an invoice number or some other identifying mark, not just the amount owed.
I might say "We have received two invoices from [name of company], both for £XXX. It is not clear if they are unrelated invoices or if we have been invoiced twice for the same bill."
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