Why Benin asks for a second mark
The apostille is a shortcut between countries that agreed to trust one another's stamps. Where that deal is not in place, the older method still stands: an officer of the receiving state looks at the file and signs for it. Document legalisation for Benin works that way, which is why the British slip is a step on the road rather than the end of it.
Benin's position on the Hague Convention
Benin has not joined the Hague Apostille Convention. That single line decides the shape of the whole job: an English paper meant for Cotonou requires embassy legalisation, whatever the British side has already put on it. The United Kingdom is inside the treaty, so an apostille is issued here, but a treaty needs both ends to be inside it before one slip can do the work of two.
From a Sheffield register to a Cotonou desk
The British half is the tidy half. One office covers the country and it works by post, so nobody in Sheffield queues for this. What you send it has to be a paper it can trace: a register copy, a court copy, or a record signed by someone whose signature it already holds on file.
The consular half is slower and less predictable. A Sheffield apostille bound for Benin depends on a desk that handles a small volume of British paper, so a file can sit while it waits its turn. Sending a spare certified copy at the same time is the cheapest insurance on this route.
French text, and what people send
Benin runs its public business in French. An English birth certificate with two stamps on it will still land on a desk where nobody has to read English, so a French version normally travels with it. Have the words done after the stamps rather than before, so the translation covers the marks as well as the original text.
Most of what leaves Sheffield for Benin is personal rather than commercial. Birth and marriage records dominate, because family and nationality questions turn on them. Where an apostille from United Kingdom to Benin is wanted for work, it is usually a degree or a police record behind a teaching, medical or aid posting.