Package-level documentation alias collides with others without warning

[netique]

When user wants to build package-level documentation page and there is a function with the same name (which could be quite common), roxygen2 generates two .Rd pages with the same \aliases. This could be a foundation of many problems (see issues https://github.com/r-lib/pkgdown/issues/1459 and r-lib/downlit#73 for real-life reference).

usethis::use_package_doc()

I do not know how to solve it, but a user should be warned at least of possible consequences. There could be option to use different alias as well.

[jennybc]

Duplicate of #1170, which concludes the best place the address this definitely is in roxygen2