When I try to download from some site, IDM show me error message that this site does not allow requesting a link second time. How should I use IDM with this site?
2. Some sites/servers do not allow requesting the same file twice. The browser made the first request of the file and then when IDM tried to request the same file again, the site sent a web page to IDM instead of the download. That is the reason for the error message.
To download from such sites try to enable "Start downloading immediately while displaying 'Download File Info' dialog" option in "Options→Downloads" dialog (if it is disabled).
If it does not help, you can set a special key in "IDM Options→General→Keys...". Press and hold this key while clicking on a download link/button so that IDM could take over the download before the first browser request. Note that "Ins" key should work for most browsers.