Repository: youtube-dl
Issue: [youtube] Extra subtitles being overwritten
Labels: subtitles

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## Checklist

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Carefully read and work through this check list in order to prevent the most common mistakes and misuse of youtube-dl:
- First of, make sure you are using the latest version of youtube-dl. Run `youtube-dl --version` and ensure your version is 2019.05.11. If it's not, see https://yt-dl.org/update on how to update. Issues with outdated version will be REJECTED.
- Make sure that all provided video/audio/playlist URLs (if any) are alive and playable in a browser.
- Make sure that all URLs and arguments with special characters are properly quoted or escaped as explained in http://yt-dl.org/escape.
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- [x] I'm reporting a broken site support
- [x] I've verified that I'm running youtube-dl version **2019.05.11**
- [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 issues including closed ones


## Verbose log

<!--
Provide the complete verbose output of youtube-dl that clearly demonstrates the problem.
Add the `-v` flag to your command line you run youtube-dl with (`youtube-dl -v <your command line>`), copy the WHOLE output and insert it below. It should look similar to this:
 [debug] System config: []
 [debug] User config: []
 [debug] Command-line args: [u'-v', u'http://www.youtube.com/watch?v=BaW_jenozKcj']
 [debug] Encodings: locale cp1251, fs mbcs, out cp866, pref cp1251
 [debug] youtube-dl version 2019.05.11
 [debug] Python version 2.7.11 - Windows-2003Server-5.2.3790-SP2
 [debug] exe versions: ffmpeg N-75573-g1d0487f, ffprobe N-75573-g1d0487f, rtmpdump 2.4
 [debug] Proxy map: {}
 <more lines>
-->

```
youtube-dl.exe --all-subs --skip-download --verbose -f303+251,299+140 https://www.youtube.com/watch?v=jc5DlOkOcU4
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--all-subs', '--skip-download', '--verbose', '-f303+251,299+140', 'https://www.youtube.com/watch?v=jc5DlOkOcU4']
[debug] Encodings: locale cp1252, fs mbcs, out cp437, pref cp1252
[debug] youtube-dl version 2019.05.11
[debug] Python version 3.4.4 (CPython) - Windows-10-10.0.18362
[debug] exe versions: ffmpeg N-92573-gc047901012, ffprobe N-92573-gc047901012, rtmpdump 2.4-20151223-gfa8646d-LibreSSL_2.5.4-x86_64-static
[debug] Proxy map: {}
[youtube] jc5DlOkOcU4: Downloading webpage
[youtube] jc5DlOkOcU4: Downloading video info webpage
[info] jc5DlOkOcU4: downloading video in 2 formats
[info] Writing video subtitles to: M2 - Complete Works _ MY LIFE IN GAMING-jc5DlOkOcU4.en.vtt
[info] Writing video subtitles to: M2 - Complete Works _ MY LIFE IN GAMING-jc5DlOkOcU4.ja.vtt
[info] Writing video subtitles to: M2 - Complete Works _ MY LIFE IN GAMING-jc5DlOkOcU4.es.vtt
[info] Writing video subtitles to: M2 - Complete Works _ MY LIFE IN GAMING-jc5DlOkOcU4.en.vtt
[info] Writing video subtitles to: M2 - Complete Works _ MY LIFE IN GAMING-jc5DlOkOcU4.ja.vtt
```


## Description

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https://www.youtube.com/watch?v=jc5DlOkOcU4 contains two sets of English subtitles (not auto-generated). One is a full subtitle, the other is a translation of the Japanese parts of the video. Unfortunately one overwrites the other as they are named the same by youtube-dl.

![image](https://user-images.githubusercontent.com/159567/58022162-2f240280-7b50-11e9-85e3-faa98c9cd6d9.png)