Don't install duplicates

[stephenmathieson]

If both user/foo.c and user/bar.c depend on user/qax.c, qax.c will be install twice.

we can avoid this by checking if ./qax/package.json exists

[Isty001]

Maybe a semantic version resolving could be a more sophisticated way of avoiding this issue, and some other too. Actually this is the only thing I miss from clib. Were there any plans/ideas to implement it in the past?