Double strokes

You've got a couple possible approaches here. 

- Duplicating and manually offseting the timing is indeed the most straightforward one. 

- As Mylenium suggested, the valueAtTime() expression is perfect for this, though it would still involve duplicating your shape group. If this is something you're doing often, you could take the time to build a little reusable rig for yourself, which you could then save as a preset. Paths can be pickwhipped so you'd only need to update one of them. Your second stroke would be pickwhipped to the first, but offset via valueAtTime (we can provide guidance if you can't find some on your own). I'd personally recommend connecting the offset amount to a slider control for easy access. In the future, you'd simply call up the preset, then either manually adjust the path, or copy/paste a new path onto the Path property within the preset. 

- You can create a procedural effects stack using Echo, you just have to work out the compositing. This is a quickie version that actually uses Echo "in reverse," so the leading (white) version is actually happening before the Trim Path keyframes in this example. The Simple Choker cleans up any rough edges and the Tint gives you color control AFTER the compositing happens. Again, this could be saved as a preset and easily called up in the future. If your shape is otherwise moving, this solution might not be as good, as Echo will also apply itself to layer transforms, etc. unless you precompose this. 

You could write some expression using valueAtTime() to be applied to the second stroke to eliminate having to fix the keyframe timing, but due to the limitations of how this works you'd still have to work with duplicate groups and adhere to a specific structure. Otherwise your example looks like a typical case for a particle plug-in drawing the stroke as a trail, so if you have particular it may be worth giving that a whirl.

Mylenium