When I play the videos in https://commons.wikimedia.org/wiki/Category:Neue_Ehrenamtliche via the TMH player (not the player embed on the file description page), the files are sometimes loaded in there entirety twice.
I was only able to reproduce this in Firefox (Firefox 50.1.0 on Linux with ffmpeg 3.1.6).
When playing File:Tutorial 1 Benutzerseite erstellen.webm via the player embedded on https://meta.wikimedia.org/wiki/User:Denis_Schroeder_(WMDE)/UltimateVideoSandbox: