Not sure if this is the cause of your issue, but I have had this happen sometimes when I click the Play button as quickly as possible when the page loads (maybe especially when it's set to the YouTube player?). It's like clicking Play causes the video player to start, but then once the page fully loads the site starts autoplaying the video on a second copy of the player that's not visible, or something. Refreshing the page and waiting a few seconds before clicking Play usually avoids it for me.
I get the same issue. For example, I'll start a video I had previously watched part way. The video will pick up where it left off, but I will also hear secondary audio of the video starting from the beginning. As @cloudymusic mentioned, waiting a second does seem to help with the issue. I've noticed that I only get this issue when the video uses the YouTube player.
Same issue on chrome 77.0.3865.90, happens mostly with the youtube player, because clicking play doesn't remove the thumbnail image so a video starts playing, then clicking again removes it but starts another video so you get 2 videos playing. Have to reload or wait some time before the page loads.
Me also too. It's like the local video starts playing but a preview for the Youtube version is sitting on top of the playing video. So then you click that and the youtube video plays on top of the GB site player. My solution has been to follow the youtube link over to Youtube and just watch it from there.
I've also run in to this, probably 50% of the time I load a video it happens. This occurs both when resuming position and on videos I haven't started. Chrome (multiple versions, this has been an ongoing issue). Only appears to happen with the youtube player as far as I can tell.