NPM doesn't reinstall dependencies that are already above it in the tree. If A depends on B and C, and B depends on (an overlapping version range of) C and D, then B and C will be installed first in A's node_modules directory, and then NPM will install the dependencies of B and C in their own node_modules directories, except that it will avoid installing a copy of C in B's node_modules directory because it already exists in a higher folder.
However, if B and C both depend on D, then NPM will install D in each's node_modules folders. You can run `npm prune` (or `npm dedupe`? I can't remember) to make it lift D up a folder so it only needs one copy.