Fix insertion fail in ["*X", "*XX"] cases

Consider a word, and a copy of that word, but with the last letter repeating twice. (e.g., ["ABC", "ABCC"])
When adding the second word's last letter, it only compares the previous word's prefix—the last letter of the word already in the Radix Tree: 'C'—and the letter to be added—the last letter of the word we're currently adding: 'C'. So it wrongly passes the "Case 1" check, marks the current node as a leaf node when it already was, then returns when there's still one more letter to add.
The issue arises because `prefix` includes the letter of the node itself. (e.g., `nodes: {'C' : RadixNode()}, is_leaf: True, prefix: 'C'`) It can be easily fixed by simply adding the `is_leaf` check, asking if there are more letters to be added.

- Test Case: `"A AA AAA AAAA"`
  - Fixed correct output:
  ```
  Words: ['A', 'AA', 'AAA', 'AAAA']
  Tree:
  - A   (leaf)
  -- A   (leaf)
  --- A   (leaf)
  ---- A   (leaf)
  ```
  - Current incorrect output:
  ```
  Words: ['A', 'AA', 'AAA', 'AAAA']
  Tree:
  - A   (leaf)
  -- AA   (leaf)
  --- A   (leaf)
  ```

*N.B.* This passed test cases for [Croatian Open Competition in Informatics 2012/2013 Contest #3 Task 5 HERKABE](https://hsin.hr/coci/archive/2012_2013/)

Author: furthermares

data_structures/trie/radix_tree.py

@@ -57,7 +57,7 @@ def insert(self, word: str) -> None:
         """
         # Case 1: If the word is the prefix of the node
         # Solution: We set the current node as leaf
-        if self.prefix == word:
+        if self.prefix == word and not self.is_leaf:
             self.is_leaf = True
 
         # Case 2: The node has no edges that have a prefix to the word