The Witcher has done to me like this a few times. What I did was click cancel, restart Steam and the need to download again is gone. Not sure if it works with Total War games, but then my guess it's Steam issue so it should work.
If you find Steam want to redownload gigabytes of data again, just cancel and restart Steam (or better restart pc). At least on my previous experience (with The Witcher) it solves the issue.