The Match Date checkbox is above the duplicates list browser. It only has any effect if the Find Duplicate Files option is enabled.
This option controls how duplicate files are detected.
If the checkbox is not checked, then the last-modified date of every file is ignored when looking for duplicate files.
If this checkbox is checked, then the last-modified date of any duplicate files must also match before the files will be recorded as duplicates.
Note that when files are copied, the last-modified date of the file is also copied, so this option is useful if you want to look for files that are not only duplicates, but which were also probably copied from the same original file.