Added find duplicate characters problem in python string section.

[NeerajBassi]

PR Checklist:

• My submission is formatted according to the guidelines in the contributing guide
• My addition is on refer on the language README.md file
• My addition does not have a spelling problem
• My submission has a proper and user-friendly description of the algorithm
• My submission has the time complexity of the algorithm
• My submission has sample input-output of the program (NOT FOR PYTHON)

What kind of change does this PR introduce? (check at least one)

• Bugfix
• New algorithm
• Optimization in previous algorithms
• Code style update
• Refactor
• Documentation
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Briefly describe the changes in this PR

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[welcome]

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[NeerajBassi]

I have added a problem about finding duplicate characters in the string category of python language.
Please review my code and merge the PR request.
Thanks.

[NeerajBassi]

Updated checklist