I have seen a bug in the most recent Safari under El Capitan. Not exactly what you are describing, but?
When I am on Amazon and try to watch a video presentation associated with a product, it will play just fine in the small window, but if I expand the window to full screen, it will play the video full screen, but the sound track will be doubled. Starting the movie in the small window will start the sound track, and expanding it to full screen will start a second playing of it from that moment, with whatever delay there was between the two actions.
Crazy that Apple can't keep from introducing as many bugs as they swat.
![Goofy [goofy]](https://athena.trixology.com/Smileys/default/smilegoofy.gif)