Since developers are already hacking eclasses to do that, it may be a good idea to just make it a standard.
The concept for EAPI5:
1) every eclass will be sourced just once;
2) any future call of inherit() with the same name will only cause inherited eclasses listed to be reordered;
3) effective exported functions will be chosen from ones exported by last eclasses on the list (last passed to inherit rather than last sourced).