Improve doctest coverage for strings/remove_duplicate.py

[subhobhai943]

Summary

Contributes to #9943 by significantly improving test coverage for the remove_duplicates function in strings/remove_duplicate.py.

Changes Made

• Enhanced doctests: Added comprehensive test cases covering various edge cases and scenarios
• Improved documentation: Better function description with Args and Returns sections
• Comprehensive coverage: Added tests for:
  • Empty strings and whitespace-only strings
  • Single word inputs
  • All duplicate words
  • Mixed case handling (case-sensitive behavior)
  • Numbers and special characters
  • Single character words
  • Mixed content scenarios

Before

The function had only 2 basic test cases:

• Basic duplicate removal
• Multiple spaces handling

After

The function now has 10+ comprehensive test cases covering:

• Edge cases (empty strings, single words)
• Boundary conditions (all duplicates, mixed case)
• Real-world scenarios (numbers, special characters, mixed content)

Testing

All doctests pass and provide comprehensive coverage of the function's behavior, ensuring the implementation is robust and well-tested.

Contributes to #9943