***Update:***  
*If this answer seems out of place, the original question was* "How do I Duplicate Keyframes?"

-----
Probably the easiest way is to the [*Dope Sheet* editor][1]. There are several ways to get there:

* Click the icon on the bottom left corner of the 3D view and select *Dope Sheet*

 ![enter image description here][2]

* Click on the dropdown that says *Default* on the *Info bar*, and select *Animation*. (Or press <kbd>Ctrl</kbd><kbd>Left Arrow</kbd> twice)

 ![enter image description here][3]

 This will take you to the [*Animation layout*][4], with a [*3D view*][5], [*Graph editor*][6], and *Dope sheet*.



From the *Dope sheet* or the *Graph editor*, select keyframes, and press <kbd>Shift</kbd><kbd>D</kbd> to duplicate.

Looping in the BGE:
==
Once you have an animation that ends where it starts, you can make it loop in the Game Engine with *Logic Bricks*:  

1. Go to the *Game logic* editor (<kbd>Ctrl</kbd><kbd>Right Arrow</kbd>)

1. Add a *Keyboard Sensor* (*Add sensor > keyboard*) and set the key by pressing the button next to the word *Key*.

2. Add an *Action actuator* (*Add actuator > Action*) and set the *Action* value to your walk animation, set the *end* value to the last frame of your animation, and set the *Action playback type* to *Loop End*.

3. Connect the logic bricks by dragging the dot next to the *Sensor* to the dot next to the *Actuator*. An *And Controller* should be automatically created between them:


![enter image description here][7]




  [1]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Animation/Editors/DopeSheet
  [2]: https://i.sstatic.net/tKmrp.png
  [3]: https://i.sstatic.net/Rm17m.png
  [4]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Interface/Screens#Screens
  [5]: http://wiki.blender.org/index.php/Doc:2.6/Manual/3D_interaction/Navigating/3D_View
  [6]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Animation/Editors/Graph
  [7]: https://i.sstatic.net/sqBdO.png