Status Report should flag duplicate module installs

## Description of the need

Modules can be installed in the root of the modules folder or in a subfolder. This makes it possible to install multiple copies of the same module onto one Backdrop system. Backdrop will choose one of these modules to make live, but it's (at best) not clear which one it has chosen, and a user can easily become confused, thinking a different module is loaded than the one that really is.

## Proposed solution

A test can be added to the status report which would alert the administrators that duplicate modules are installed.

## Alternatives that have been considered

Another option would be to allow duplicate modules to appear multiple times on the modules page.

## Additional information

Add any other information or screenshots that would help.

## Draft of feature description for Press Release (1 paragraph at most)

The Status Report now flags duplicate modules.