Keyframes, just like about everything in Blender, can be copied and reused. In the following steps, you will learn to copy a keyframe and place it elsewhere in an F-Curve:
Make sure that the arrow button in the Graph Editor header is darkened, so that only the channels related to selected items are displayed.
Select the cube in 3D View.
In Graph Editor, click on the triangle next to the CubeAction channel to open it and repeat with the Location channel. Click on the word Y Location in the menu to select the F-Curve. Select the keyframe for the Y Location at frame 20 with the RMB.
Press Ctrl + C to copy the keyframe.
Move the current frame indicator to frame 14. Press Ctrl + V to paste the keyframe.
Press X to delete the keyframe you copied. Press Ctrl + Z to undo the deletion.