sorry bazza, may i borrow your screenshot ? 

but that timeline slider is important
first, move it to desired frame where you want to put your copied/duplicated frames in
say, you want to copy pose of frame 0 to frame 48, then point that slider to frame 48
or if it’s too hard to slide that line-thingy, you can use this box

just type the time in there like this : [frame_number]f and Enter (in this case, 48f)
or if there’s seconds/minutes
[minute]m [second]s [frame_number]f
then do what bazza did in his screenshot
- select a layer (even group layer is fine) that has the desired waypoint
- right click that waypoint then choose duplicate
same goes to keyframes, but you don’t need to look for any specific layer
just set the slider, right click on desired keyframe, then duplicate
oh, and also waypoint and keyframes are different
yes, it’s synfig’s official terminology (weird, huh…?) 
