if you already downloaded a file, but want to download it again (maybe you accidentally deleted it), mldonkey tells you 'file already downloaded'. after this, type 'force_download'.
~> dllink ed2k://....
file already downloaded
(the download starts nevertheless)
mldonkey uses the old_files list to detect already downloaded files. So, you can also remove hashes from that list (while the core isn't running) to avoid using the force_download command.
If you use the webinterface, you can't use force_download if you clicked a link in a search result window (even though the error message indicates that should do that). Rather, you have to figure out the ed2k-URL (how do you do that from the search window???) and in the input field enter
dllink <the URL>
and then use force_download in the input field.
Now if just someone could explain how you figure out the URL from the search window...