When attempting to download video+audio, youtube-dl will falsely claim "already downloaded and merged" if the video has already been downloaded

Checklist

[x] I've verified that I'm running youtube-dl version 2019.06.27

Joshuas-MacBook-Pro:ff7 joshuarodman$ youtube-dl -U
Updating to version 2019.06.27 ...
Updated youtube-dl. Restart youtube-dl to use the new version.

[x] I've checked that all provided URLs are alive and playable in a browser
[x] I've checked that all URLs and arguments with special characters are properly quoted or escaped
[x] I've searched the bugtracker for similar bug reports including closed ones
[x] I've read bugs section in FAQ

Verbose log

Joshuas-MacBook-Pro:ff7 joshuarodman$ youtube-dl -v -f 243+250  'https://www.youtube.com/watch?v=tht4GQWOHiE&list=PLE3sw8KYN43fG6U6S0c00PLZtzI_dftv-'
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'-f', u'243+250', u'https://www.youtube.com/watch?v=tht4GQWOHiE&list=PLE3sw8KYN43fG6U6S0c00PLZtzI_dftv-']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.06.27
[debug] Python version 2.7.16 (CPython) - Darwin-17.7.0-x86_64-i386-64bit
[debug] exe versions: ffmpeg 4.1.3, ffprobe 4.1.3, rtmpdump 2.4
[debug] Proxy map: {}
[youtube:playlist] Downloading playlist PLE3sw8KYN43fG6U6S0c00PLZtzI_dftv- - add --no-playlist to just download video tht4GQWOHiE
[youtube:playlist] PLE3sw8KYN43fG6U6S0c00PLZtzI_dftv-: Downloading webpage
[download] Downloading playlist: Final Fantasy VII - Let's Play/Walkthrough (2019) - w/ Cam, Commentary & Live Chat! (Complete)
[youtube:playlist] playlist Final Fantasy VII - Let's Play/Walkthrough (2019) - w/ Cam, Commentary & Live Chat! (Complete): Downloading 7 videos
[download] Downloading video 1 of 7
[youtube] tht4GQWOHiE: Downloading webpage
[youtube] tht4GQWOHiE: Downloading video info webpage
[download] Final Fantasy VII - Livestream _ Casual Playthrough_Let's Play _ Cam & Commentary _ P1!-tht4GQWOHiE.webm has already been downloaded and merged
[download] Downloading video 2 of 7
[youtube] j_iIvithn3s: Downloading webpage
[youtube] j_iIvithn3s: Downloading video info webpage
[download] Final Fantasy VII - Livestream _ Casual Playthrough_Let's Play _ Cam & Commentary _ P2!-j_iIvithn3s.webm has already been downloaded and merged
[download] Downloading video 3 of 7
[youtube] n3e08HcccPg: Downloading webpage
[youtube] n3e08HcccPg: Downloading video info webpage
[download] Final Fantasy VII - Livestream _ Casual Playthrough_Let's Play _ Cam & Commentary _ P3!-n3e08HcccPg.webm has already been downloaded and merged
[download] Downloading video 4 of 7
[youtube] LpZxNW0Hk30: Downloading webpage
[youtube] LpZxNW0Hk30: Downloading video info webpage
[download] Final Fantasy VII - Livestream _ Casual Playthrough_Let's Play _ Cam & Commentary _ P4!-LpZxNW0Hk30.webm has already been downloaded and merged
[download] Downloading video 5 of 7
[youtube] s8j6tdKCEVc: Downloading webpage
[youtube] s8j6tdKCEVc: Downloading video info webpage
[download] Final Fantasy VII - Livestream _ Casual Playthrough_Let's Play _ Cam & Commentary _ P5!-s8j6tdKCEVc.webm has already been downloaded and merged
[download] Downloading video 6 of 7
[youtube] fXtJg-0QnpI: Downloading webpage
[youtube] fXtJg-0QnpI: Downloading video info webpage
[download] Final Fantasy VII - Livestream _ Casual Playthrough_Let's Play _ Cam & Commentary _ P6!-fXtJg-0QnpI.webm has already been downloaded and merged
[download] Downloading video 7 of 7
[youtube] ZXUYieiQQH4: Downloading webpage
[youtube] ZXUYieiQQH4: Downloading video info webpage
[download] Final Fantasy VII - Livestream _ Casual Playthrough_Let's Play _ Cam & Commentary _ P7!-ZXUYieiQQH4.webm has already been downloaded and merged
[download] Finished downloading playlist: Final Fantasy VII - Let's Play/Walkthrough (2019) - w/ Cam, Commentary & Live Chat! (Complete)

Description

I stumbled when trying to navigate the new reality of youtube where

video and audio are always split
typical best formats are way way way bigger than I want.

Thus I'm struggling to use the various format flags and making missteps, leading this this situation:

Step 1 - Download video-only version of playlist accidentally with -f 243 Step 2 - Attempt to download video + audio with -f 243+250

This results in youtube-dl deciding that everything has been downloaded and merged, but yet there is no audio. The workaround is to delete all the files and download again.

I don't know enough about the project to know how hard it would be to either just work, or to give an accurate output message (eg, cannot use already-merged webm to create merged audio+video)

Feel free to convert to feature request etc as makes sense.

ytdl-org / youtube-dl