That’s a question we recently received, so we thought we’d devote a post to answering it!
When French developed from Latin, it retained a gendered marking for the majority of its names — marking which is most obvious when looking at masculine/feminine pairs of names. In Old French, the most common way of feminizing a name was simply adding an -e to the end, but in Middle French, especially by the early 15th C, it became more common to duplicate the final consonant and then add -e.
Some names, however, (particularly ones that already ended in -e!), were used in the same form by both men and women. Here are some examples:
Claude was used in this form by both men and women in France in the 16th C.
While the more common masculine form of the name was Dennis, both Denise and Denyse were used by both men and women at the end of the 13th C.
Gile, which can be a form of Giles or sometimes Gilo, is also the Old French vernacular form of the feminine form of both names, found in the early 14th C.
A bit of a rarity, but Guillaume was used by both men and women in the early 14th C.
Laurence was the usual Middle French vernacular for both men and women in the 16th C.
While Marin was almost exclusively masculine in the 16th C (and the expected Middle French feminine form would be Marine), we have one lone example of Marin used by a woman.
Phelippe, Phlippes, and Phlippe were all used by both men and women; the first form is an Old French one found in the early 14th C, while the latter two are 16th C Middle French forms.
Finally, Robert. Diminutive forms such as Robine and Robinette were much more commonly used by women, but Robert itself was used, albeit rarely, by both men and women.
So, there you are! These are the names we’ve found that were used in exactly the same spelling by both men and women in Old and Middle French.