You have found the 'Add' button then, and the 'Include files and subfolders', that's a good start.
You have it set there to look for duplicate mp4 files on C: drive and in it's sub-folders - but at the moment you are only searching for those that have all 3 of the same name, same size, and same modified date.
The 'Match by' and the 'Ignore' sections are very important.
Match by sets <u>what</u> to look for as being a duplicate.
Ignore can cut out a lot of unwanted/uneeded results. (if you are searching all files and haven't specified a filetype).
For files to be considered duplicates they have to match all of the things ticked in 'Match By'. (And be none of those types ticked in 'ignore').
eg. As you have it if set there then if they match the name and size, but not the date, then they are not counted as being duplicates.
Try searching with only name ticked, that will find any/all the mp4s that have the same name.
It doesn't matter what they contain or when they were made/modified, it's only looking at the names.
If you search with just size ticked, that will find any/all the mp4s that have the same size.
and so on.
When I am seraching for duplicates of any type then I'm usually loking for files that are the same, no matter what they are called or when they were saved/modified.
So I have mine set to look for 'Content', which automatically ticks 'Size' as well.
(It's faster to check for the same size files first then only check the contents if they are the same size. because if the size isn't the same then the contents can't be the same).
Note also the ignores, I don't want to see any of those.
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So say I'm searching songs, that will find the same song that's been saved more than once or copied to a different place and/or renamed - no matter what it has been named when saving it, or what date it was saved - if the contents are the same the files are duplicates.
If you could tell us just what kind of duplicates you are wanting to find them we may be able to give tips on the best way to go about that.