IDM shows a video download panel at some site, but when I start a download, it shows error message that this video file cannot be requested twice. How can I download from this site?
Some sites generate temporary links for video and do not let to request them twice.
To download from such site you should do the following:
2. Open "Options→General" IDM dialog and press "Keys" to customize keys. On a new dialog enable checkbox "Use the following key(s) to force downloading with IDM for any links:" (arrow 1 on the image). Select "Ins" key (arrow 2 on the image), then DISABLE checkbox "Check for left mouse button clicked on a link along with the special key(s) pressed to force downloading the link" (arrow 3 on the image) and make sure that checkbox "While holding down a special key DO NOT take over the downloads which are web-pages, pictures, scripts and etc." is enabled (arrow 4 on the image).
When web page with player will start loading video, press "Ins" key (you should press it before clicking "Play" button in web player) and hold it until IDM catch video download.