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BeeTee-Ess - your IDM process is the same as mine is with FDM, with one minor difference, I uncheck the run calibre now, so I can copy the msi to my installed software folder before I forget, and then I run calibre from the taskbar.
I also click the Advanced button on the License screen, I don't change anything (I turned off create desktop shortcut years ago) just like to know nothing's changed. But I would bet London to brick on, that me doing that would not make the slightest bit of difference, likewise with me not running calibre immediately after the install.
I think Hidden.Platypus solved his similar issue by using a proxy, the issue seems to arise when successive calibre releases are the same size.
It would seem that some blithering idiots configure their pipeline nodes to compare file sizes and sender location, and if they're similar then then they deliver what's in their cache with the new name. Hello out there that's why checksums were invented. The blithering idiot is almost certainly your ISP.
Meantime line up a free proxy server in case you need it next week. The FoxyProxy add on (firefox), can be configured to always kick in for certain sites.
I'm not sure if IDM is a beefed up FDM with a price tag, or if FDM is a reverse engineered IDM, but last time I looked at them side by side they were hard to differentiate.

maybe Kovid could consider publishing the checksum on the relevant download page.
BR