On one of my dual-boot system (with Windows 10 installed as upgrade), I get two
Windows boot menu entry from os-prober.

First entry is "Windows 10 (loader)", which is properly pointing to Windows
"System partition" (with /bootmgr).
Second entry is "Windows 7 (loader)", which is the "second" Windows partition
(containing Windows installation). However, this second partition should never
be booted, since there is a "Windows system partition" already (and trying to
boot from there won't work).

As noted on
https://wiki.archlinux.org/index.php/GRUB#Windows_installed_in_BIOS-MBR_mode ,
it would be better to not add this second entry at all, if a Windows system
partition is already detected.