Changed the list of words to set of words

... and now we don't need to check if the word is duplicated, and this
generates on less error, and that means one less error to manage. WE WIN!

src/repository.rs

@@ -1,5 +1,5 @@
 use std::collections::BTreeMap;
-use std::collections::LinkedList;
+use std::collections::BTreeSet;
 use std::fs::File;
 use std::io::{Read, Write};
 
@@ -7,7 +7,7 @@ use serde_derive::Deserialize;
 use serde_derive::Serialize;
 use toml;
 
-pub type WordStorage = BTreeMap<String, LinkedList<String>>;
+pub type WordStorage = BTreeMap<String, BTreeSet<String>>;
 
 #[derive(Debug, Serialize, Deserialize)]
 pub struct WordList {
@@ -19,7 +19,6 @@ pub struct WordList {
 pub enum WordListError {
     InvalidFormat,
     InvalidWord,
-    WordAlreadyExists,
     SaveFailure,
 }
 
@@ -106,18 +105,14 @@ impl WordList {
             .to_string()
             .to_lowercase();
         let mut list = if !target.contains_key(&initial) {
-            let empty_list = LinkedList::new();
+            let empty_list = BTreeSet::new();
             empty_list
         } else {
             target.get(&initial).unwrap().to_owned()
         };
 
-        if list.contains(&word.to_string()) {
-            Err(WordListError::WordAlreadyExists)
-        } else {
-            list.push_back(word.to_string().to_lowercase());
-            target.insert(initial, list);
-            Ok(())
-        }
+        list.insert(word.to_string().to_lowercase());
+        target.insert(initial, list);
+        Ok(())
     }
 }