Skip fixup of existing files and add --fixup force to force it

yt_dlp/YoutubeDL.py

@@ -2690,7 +2690,10 @@ def fixup():
                         return
                     elif fixup_policy == 'warn':
                         do_fixup = False
-                    assert fixup_policy in ('detect_or_warn', None)
+                    elif fixup_policy != 'force':
+                        assert fixup_policy in ('detect_or_warn', None)
+                        if not info_dict.get('__real_download'):
+                            do_fixup = False
 
                     def ffmpeg_fixup(cndn, msg, cls):
                         if not cndn:

yt_dlp/options.py

@@ -1230,11 +1230,12 @@ def _dict_from_options_callback(
     postproc.add_option(
         '--fixup',
         metavar='POLICY', dest='fixup', default=None,
-        choices=('never', 'ignore', 'warn', 'detect_or_warn'),
+        choices=('never', 'ignore', 'warn', 'detect_or_warn', 'force'),
         help=(
             'Automatically correct known faults of the file. '
             'One of never (do nothing), warn (only emit a warning), '
-            'detect_or_warn (the default; fix file if we can, warn otherwise)'))
+            'detect_or_warn (the default; fix file if we can, warn otherwise), '
+            'force (try fixing even if file already exists'))
     postproc.add_option(
         '--prefer-avconv', '--no-prefer-ffmpeg',
         action='store_false', dest='prefer_ffmpeg',

yt_dlp/postprocessor/ffmpeg.py

@@ -665,7 +665,7 @@ class FFmpegFixupPostProcessor(FFmpegPostProcessor):
     def _fixup(self, msg, filename, options):
         temp_filename = prepend_extension(filename, 'temp')
 
-        self.to_screen('{msg} of "{filename}"')
+        self.to_screen(f'{msg} of "{filename}"')
         self.run_ffmpeg(filename, temp_filename, options)
 
         os.remove(encodeFilename(filename))