Why is ne ... que needed here?

Question

In the sentence, "Françoise Dupont et Fantômette n'étaient qu'une seule et même personne," why is the negation used in French. It wouldn't be needed in English.

Accepted Answer

This is no negation (note, that pas is missing to make it a complete negation). The construction ne..que is translated as "only" or "just":
Les deux n'étaient qu'une seule personne. -- The two were just one person.

Answer

But, how come the construction ne..que is even used? Why can't you say,  "Françoise Dupont et Fantômette étaient une seule et même personne," meaning, Françoise Dupont and Fantômette were one and the same person, in English.

Answer

There are always several ways to express the same thing. The ne..que version sounds a bit more elegant.