Improve doctest coverage for remove_duplicates function

Contributes to #9943

- Added comprehensive doctests covering edge cases
- Added tests for empty strings, single words, all duplicates
- Added tests for mixed case, numbers, and special characters
- Improved documentation with better examples

Author: subhobhai943

strings/remove_duplicate.py

@@ -1,15 +1,58 @@
 def remove_duplicates(sentence: str) -> str:
     """
-    Remove duplicates from sentence
-    >>> remove_duplicates("Python is great and Java is also great")
-    'Java Python also and great is'
-    >>> remove_duplicates("Python   is      great and Java is also great")
-    'Java Python also and great is'
+    Remove duplicates from sentence and return words in sorted order
+    
+    Args:
+        sentence: Input string containing words separated by spaces
+        
+    Returns:
+        String with unique words sorted alphabetically
+    
+    Examples:
+        Basic functionality:
+        >>> remove_duplicates("Python is great and Java is also great")
+        'Java Python also and great is'
+        
+        Multiple spaces handling:
+        >>> remove_duplicates("Python   is      great and Java is also great")
+        'Java Python also and great is'
+        
+        Edge cases:
+        >>> remove_duplicates("")
+        ''
+        
+        >>> remove_duplicates("   ")
+        ''
+        
+        >>> remove_duplicates("hello")
+        'hello'
+        
+        >>> remove_duplicates("hello hello hello")
+        'hello'
+        
+        Mixed case (case sensitive):
+        >>> remove_duplicates("Python python PYTHON")
+        'PYTHON Python python'
+        
+        Numbers and special characters:
+        >>> remove_duplicates("1 2 3 1 2 3")
+        '1 2 3'
+        
+        >>> remove_duplicates("hello world hello world!")
+        'hello world world!'
+        
+        Single character words:
+        >>> remove_duplicates("a b c a b c")
+        'a b c'
+        
+        Mixed content:
+        >>> remove_duplicates("The quick brown fox jumps over the lazy dog")
+        'The brown dog fox jumps lazy over quick the'
     """
     return " ".join(sorted(set(sentence.split())))
 
 
 if __name__ == "__main__":
     import doctest
 
-    doctest.testmod()
+    doctest.testmod()