The same charge, listed twice on one invoice — usually by accident, sometimes not.
Most duplicate line items come from ordinary drafting mistakes: a row gets copy-pasted while building the invoice and the quantity or date isn't updated, or invoicing software glitches and inserts the same imported record twice. Less often, it's a vendor testing whether anyone checks closely before paying. Either way, the invoice looks unremarkable at a glance — two lines with the same description, quantity, and price, sitting a few rows apart in a list of a dozen or more items.
A person scanning an invoice reads top to bottom once, checking that the total looks roughly right. Two identical rows several lines apart don't stand out the way an obviously wrong total would — the subtotal, tax, and grand total can all still add up correctly, because the duplicate is priced consistently with itself. The invoice is internally consistent and wrong at the same time.
The reliable way is to compare every line item's description, quantity, and unit price against every other line, which is exactly the kind of repetitive check software does well and people do slowly. Upload the invoice below and duplicate lines are flagged automatically.