- Hold Shift and use LMB to create your bright red selection box over your keyframe/s
- Right-click and click 'Copy'
- To paste, right-click somewhere else on the timeline and click 'Paste'
OR: (this is better!)
- Click on the keyframe you want to copy
- MMB and drag to the desired frame (this copies the same positions!)
- hit 'S' to key all