Saving BOOK file (.indb) acts as SAVE AS
Adobe InDesign v16.1 (12 Feb 2021 release)
Windows 10
This bug has appeared again, after disappearing for a couple of versions. But it was in a previous version a few months ago.
When the Book properties panel is open, I make changes to my Book file (.indb) and then close the Book panel or select "Save" from the panel menu. It asks me do I want to save.
Yes, I want to save... but the dialog box is acting like a Save As... It's asking me for a file name, then asking me to overwrite the existing file, instead of simply saving the existing file as a regular Save should do.

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Steven Conrad commented
I would like to echo that this bug be fixed. I'm surprised that not more people have complained about it. Not critical but becomes highly irritating when exposed to it day after day. Seems like a simple fix.
I'm using version 18.3 x64, updated today, May 26. 2023
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J Kaufman commented
I have experienced this exact same issue, and while it seems like a minor irritation, it is definitely a bug that should be fixed. What's the point of having distinct "Save" and "Save As" functions in the menu, if they do the exact same thing? "Save" simply isn't working as it should.