Great solution here, but one thing interests me here. Why is the variable e marked as mutable? entry(x).or_default() already returns a mutable reference.

Have a look at the triangles with short lengths. See what you notice.

Given "the" triangle of odd numbers...

https://xkcd.com/221/
You might have chosen them randomly but they remain static values that could be hard coded.

I think this one works now.

Totally agree. Fixed in this fork

Do you have an ability to approve this new fork and replace the original (taking into account that it has a breaking change)?

The solution setup has a return type of &str where the solution has type String.

fn remove_duplicate_words(s: &str) -> &str {
    s // you code here
}

As someone new to Rust this is very confusing.
Also, it would be better to remove the hashmap from the solution setup.