As opposed to NPM’s nested dependency model YARN has a flat dependency model. Does this mean there is not a single case where there would be a duplication in the packages, right? Is this always a good thing to have?
Yarn follows the same model as npm@3.
It tries to flatten packages if there are no conflicts, in that case it makes them nested.
There is an option to force flat node_modules in Yarn, then you would have to resolve package conflicts manually.