OK. I have the Windows installers fixed and uploaded (new checksums updated). If you installed Sigil-0.9.12 on Windows before the time of this post, I urge you to redownload and reinstall to avoid any entity-doubling problems. Sorry about the mistake and any confusion.
To quickly find out if you have the bad version installed: open a brand new epub (epub2) and in Code View type "©" (without the quotes) in between the p tags. If you see one copyright symbol in Preview, you're good to go. If you see two copyright symbols, youdDownload the fixed installer
from the github release page and reinstall.