Open up the layer with the keyframes in the keyframe editor. Copy all of the keyframes. Return to the Timeline. Select all of the layers by using the shift or command key while selecting all of the layers you want to paste the keyframes to. Then hit the Command and V keys to paste the keyframes.
Keep in mind it will paste the keyframes so the first keyframe starts where the playhead is at so be sure to move the playhead to where you want the first keyframe to start. Hope this helps.
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