Duplicating keyframes in After Effects
Click the layer were your made the keyframes
Press U or P to reveal the keyframes
Highlight the keyframes. You will know it's highlighted since the color of the keyframes is colored blue(see screenshot below)

CTRL + C to copy the keyframes
Move your time indicator to the next time you wanted
CTRL + V to paste the keyframe
Note: You need to properly copy the Start and End of the keyframe so that you won't mess up the animation