This page of the Settings dialog allows you to modify how the program will search for duplicates.
Base search on following fields:
Select the fields you want to use when searching for duplicate files. The marked fields must match for both candidates, before they are considered identical. Select File Checksum and File size for true duplicates, but by selecting more fields you will speed up the search. On the other hand, the more fields you select, the fewer duplicates will be found. For name only duplicates (e.g. different versions of the same file), use File name and File type only.
Other search options:
Select the options you want to activate when searching for duplicates. If you choose to use MD5 hash as a final step when comparing files, the search will take somewhat longer to finish. The default checksum (CRC32) has 32 bits and is quite accurate, since the files also have to be of the same size. It is the standard checksum used in ZIP-archives. By using MD5 hash (128 bits) you can rest assured that the files are truly identical.