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.

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.