Added firstDuplicate algorithm and tests

[LoganAaron20]

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Describe your change:

• [x] Added firstDuplicate algorithm
• [x] Created test suite for firstDuplicate function
• [x] Ensured functionality was working

Checklist:

• [ x] I have read CONTRIBUTING.md.
• [ x] This pull request is all my own work -- I have not plagiarized.
• [x ] I know that pull requests will not be merged if they fail the automated tests.
• [x ] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ x] All new JavaScript files are placed inside an existing directory.
• [x ] All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames. Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not
• [ x] All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• [ x] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.