This is an option in the 'Scan Location' tab.
Mount points and junctions are links that point to another drive or folder. If Duplicate Cleaner follows them, the same physical folder can be scanned under two different path names, making a single file appear as a pair of "duplicates". Deleting one of these false duplicates would actually delete your only copy of the file.
For this reason the "Don't follow NTFS mountpoints and junctions" setting (Advanced folder Search Options) is there as a safeguard, and we recommend leaving it enabled unless you specifically need to scan through such links (e.g. some OneDrive setups).