You might write functions that are shared and even wrap them up into a plugin, but those are all immutable. When you need to do similar things you end up copying code and tweaking, and may end up with a mess. There's a better way.
Moo4q uses MooTools to fill in where jQuery leaves off. MooTools is modular and encourages users to only use the pieces they need. We only need a couple modules from MooTools, none of which contain any code relevant to working with Elements, Animation, or AJAX. Moo4q leaves that to jQuery. So there's little, if any, functional duplication. All said and done, the MooTools code is only 8k gzipped--diddly squat.