Right clicking Add file to search for duplicates of... generates a prompt to Add Folder instead of Add File.
Double clicking, or clicking the green check icon, brings up a Windows file selection dialog. If I am searching for duplicates of a particular file then I already know what I am looking for so it seems unnecessary to have to drill down through the file selection dialog to find an instance. I'd like an option to simply enter the filename.ext without a path, which is redundant in the context of a search for duplicate files.
It's not redundant at all, unless you are only searching for duplicates based on filenames (but then you might as well use the Find tool if you only care about files with a particular name instead of all duplicate names; you can type a filename in there and get a list of all files with the same name).
If you are searching for duplicates based on file contents (MD5) or name+size then a particular file must be specified in order to find those attributes, not just a filename.