"Find similar files" settings: "use extension" unchecked, "same file data" selected.
Bugs in displaying search results in this mode are following.
1) If extension of file does not match its format (which happens sometimes when saving pictures from the Web), then it won't be displayed in top panels of window and properties won't be detected.
2) If files are not jpegs (seems like only for jpegs it works properly), then thumbnails for found copies in bottom panel of window are not created.
Brb in few hours to see how you coped with errors reported last year.
P.S. When you will try to repeat it, use Windows XP and release (not debug) executable. In Windows 10 x64 2nd bug does not occur, but Xnview classic is 32bit application, isn't it?