Single Key Macro for OS X
I suggest you try Karabiner Elements where you can do what you want (but I don't know HOW fast).
You'd have to import a VERY simple "Complex Modification" to press two key presses in a row.
Something like this:

{"title": "Make single letter click twice FAST …",
 "rules": [
        { "description": "Press 'x' twice.",
         "manipulators": [
           "conditions":[{"bundle_identifiers": [ "^com.game.name" ],
                                        "type": "frontmost_application_if" }
                         ],
                 "from": { "key_code": "x",
                          "modifiers": { "optional": ["any"] }
                         },
                   "to": [
                          { "key_code": "x" }, { "key_code": "x" } ],
                 "type": "basic"
} ] } ] }


Karabiner comes with "EventViewer" which will tell you in its tab "frontmost application" what name it expects (keep "^" !).
(HERE is an example which explains the importing into K-Elements.)