font.names() lists 'symbolic' family names guaranteed to exist.  'TkDefaultFont' is the default Text widget font on a particular OS.  IDLE replaces that with 'TkFixedFont', which is translated to the actual family name on each OS: Courier on Windows (ugh) and something like Monaco on macOS.

Thank you for the additional information.  For *nix users, removing the useless duplicates should be an improvement.  (This is aside from the test issue, which could have been fixed otherwise: perhaps by inserting 'TestFont' at the top of the list and making other adjustments.)