Though I have visited the old watering hole (Sloppy Joe's) of Papa's in Key West with my papa, the etymology of the "Papa Doble" (aka Hemingway cocktail aka Hemingway daiquiri) is from his favorite bar in Havana called La Floridita.
There are many variations of this cocktail, but the following recipe is the one I use and love. I have made it for many of my friends, and they have all enjoyed sipping this rum-filled drink on a warm summer day. I haven't had one since last summer with Moo...I associate them with her now and miss her tonight as I sip away. I made one tonight (11:22pm) - it was a long day, so what else is a child to do but rely on her Papa for some support after a grueling 16 hour work day?
Read Moo's blog here - my dear is in Africa: http://ngogo.blogspot.com/
1 1/2 oz Light rum
1/4 oz Maraschino liqueur (this can be hard to find, but you need to search until you do/its essential!)
3/4 oz Lime juice
1/4 oz Grapefruit juice
Shake it up with ice and pour into a martini glass.
Now go re-read The Sun Also Rises!