You can if you add a #1 to the end of the video (assuming it’s YouTube). Or #2, or increment it further. Also, hide the preview if you are posting on a desktop browser. (Mobile browsers don’t have a preview.)
All of this is covered extensively in the thread that ECG posted already, which is in the Site Feedback category. (Site Feedback is where you discuss the forum software, ATMB is where we talk about posts and threads, this thread doesn’t really belong here.)
I do think it would be helpful for a video posting FAQ, maybe a sticky. It sucks that we need one, but Discourse is so freaking buggy it is absolutely necessary.
I’ve tried the share Youtube link with added / method described in this thread, but it doesn’t work for me.
The only thing that works for me is the paste from Notepad and speedy click on Reply. However, if I’m too late and the preview is created, it becomes impossible to post a Youtube video, even if I try doing it in a different thread or log out from the site and back in.