"Duffel" or "duffle"

Solution 1:

Interesting. According to Wikipedia both spellings are correct

A duffel bag (or duffle bag, kit bag, gym bag) is a large cylindrical bag made of cloth (or other fabric) with a drawstring closure at the top.

On the other hand Google NGram shows, that first spelling is more common in the last couple of decades. So just to be safe I'd use the duffel bag spelling.

P.S. Funny thing is that there are both Duffel Bags and Duffle Bag company and both are producing... that same bags :)

Solution 2:

CALD and OALD list both spellings as possible with duffel as the primary entry. Etmyonline says of duffle

1670s, from Du. duffel, from Duffel, town in Brabant where the cloth was originally sold. Duffel bag is Amer.Eng., first recorded 1917 in a letter of e e cummings.

It probably started off with duffel and later duffle came to be used as well.