It can be quite common (also on Linux). In my case you edit something in the first modal dialog. You can though bring up a second dialog to examine the state of something you want to select or not. Technically this could be allowed to float around but it makes more sense to pop it up to check something then pop it down and continue editing. Breaking modal flow can potentially allow the user to make edits contradict each other. Personally I’m fine with modal-in-modal if it makes sense and prevents me from making mistakes.
What really frustrates me, is that you can’t move a window, which is behind a modal dialog box, while that modial dialog box is open. If that was possible, then modal-in-modal would probably not be as annoying for me.
On Linux, I believe it is possible to move the window, which is behind a modal dialog; maybe that is why I did not get annoyed there.
So my requirement for modals (in general), is that it allows you to move any window behind it; perhaps by CMD-dragging that window.
Still a stack of 12 modals-in-modals is really bad design (or malware). 
I don’t think under Linux (or Windows for that matter) you can move a modal blocked window. I think though in good UI design the second dialog must contain enough information so you don’t need to peek at the underlaying dialog.
Sadly, reality is that I often need to move a modal, to see a filename or some other information.
-Yes, there are desktop systems, which allow you to move windows under the modals. Mac OS X allowed it (though normally a sheet was presented instead of a modal, but modals were possible - modals were normally not needed though)
Edit: Just tried this under Linux:
Open Mousepad, type “test”, click “Save As” and you can move the “Save Panel” (which is modal), then you can move the window under it as well, while it’s active, so yes, a “parent” window can be moved while a modal is open, so you can see filenames and other information under Linux.
Maybe it’s not possible under Windows, but I don’t care, because I don’t use Windows and never will feel comfortable in that environment.
Depends on window manager? Or toolkit? Or application? Fedora Workstation, GNOME wayland, repeat you test with gedit: you can move both windows as a set but not relative to each other. Same system, repeat your test with FontForge: you can move both windows however you like.
Possibly. What’s important is that it’s convenient and there’s an example of what I’m trying to say. I’m definitely not saying that Linux got the desktop environment right - far from it (it’s one of the main reasons I’m moving to Haiku).