### What it does
Checks for files that are included as modules multiple times.

### Why is this bad?
Loading a file as a module more than once causes it to be compiled
multiple times, taking longer and putting duplicate content into the
module tree.

### Example
```
// lib.rs
mod a;
mod b;
```
```
// a.rs
#[path = "./b.rs"]
mod b;
```

Use instead:

```
// lib.rs
mod a;
mod b;
```
```
// a.rs
use crate::b;
```