I was able to reproduce the problem by downloading the file twice over a short period of time. (Looks like <60 seconds).
Check your default download directory and see if you actually have a copy of the file there. On Windows that would be [C:\Users\<UserName>\Downloads].
I also noticed that if I reached this page:
If I 'click' the black triangle to the left of the file name, it initiates a download of that file. If I then click the file name, I'm brought to a download screen. If I then 'click' "Download", then I get the message because I already downloaded it once. The same occurs if I attempt to 'click' Download twice.
If you don't have the file in your 'Downloads' section, then wait and ride the timer down to zero and then attempt to download again.
If you're able to download after riding the timer down, note if you're prompted to 'overwrite' the file. That would indicate that the file already exists in the target directory as well.