For some videos, while syncing or editing subtitles, you may see that the video player displays two copies of subtitles, which may be in the same or different languages:
This happens when the video you are working with - usually from YouTube or Vimeo - already has closed captions at its host provider and your Web browser's settings enable displaying those subtitles. You can easily turn these subtitles off at any time.
To turn off closed captions for a YouTube video (see above), click the Gear icon to the right from YouTube CC icon in the bottom right corner of the video player and select Subtitles/CC -> Off.
To turn off closed captions for a Vimeo video (see below), click on the blue Vimeo CC symbol in the video player and select None from the radio button selector: