Suggestion: sort the User Plug-ins - Not installed list with a left click on Plug-in Name column header, then it'll be easier to find it - by default the list is sorted on latest to earliest Release Date.
The plug-in name is "Find Duplicates" so you should find it in the 'F's. If its not there then you probably already have it, change the filter to Installed to see what you have.
BR
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