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I installed Fedora 31 in a dual boot scenario with Windows 10. Initially the boot menu showed four choices then after a few days a new entry got added, the top one in the image below.

Can anybody please tell me what is going on here? Incidentally I can boot into both fedora instances just fine (and Windows too):

Boot menu

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The top three entries refer to the same instance of Fedora 31 OS, but using different kernel versions (and in case of the rescue mode, different boot options and perhaps a special rescue-oriented initramfs file). Fedora 31 simply received a kernel update from kernel version 5.3.7-701.fc31 to version 5.5.5-200.fc31. You can check the currently-running kernel version with uname -r: no root permissions needed.

Whenever a kernel update is installed, the old kernel is not immediately removed: you should never remove the kernel package that matches the kernel version you're currently running unless you really know what you're doing, and the package management system will usually seriously discourage you from doing that.

Activating a new kernel update will require a reboot (or a kexec operation, which will usually be almost equivalent to a reboot). By installing the new kernel alongside the old, the package management system won't have to force you to reboot the system immediately once the kernel update has been installed, but allows you the option to postpone the reboot for as long as you want/need.

The package management system will typically automatically keep a maximum of about three kernel versions at hand (adjustable) so that if the newest kernel turns out to not work for you, you can easily reboot to the previous kernel version to remove the non-working kernel package.

This may be important if your system needs to have third-party drivers for basic functionality like display or storage controllers, and those drivers have not been integrated with the kernel package management system using DKMS or similar: in those cases, you would need to take some manual steps to rebuild the third-party driver manually for the new kernel version before booting to that version. Forgetting to do that would cause the attempt to boot with the new kernel to fail, until you go back to the older kernel, run the required steps for the new kernel version, and then boot to the new kernel.

But if your system runs well on standard drivers already included to the kernel packages, or on well-integrated third-party drivers, you probably won't have to worry about any of the above: just use the top-most boot option to boot into Fedora unless you find you have problems with the kernel or device drivers.

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You updated the kernel. By default, Fedora keeps the last four installed kernels around (really, the running one and three more) when updating. A newly installed machine has just the running kernel (and the fallback/troubleshooting entry), when you get the fifth the oldest one that isn't the running one is erased. That ensures that in case of some catastrophic failure of an update kernel you do have clean fallbacks. (Yes, it does happen that some critical driver or other component fails or malfunctions. Not often, by a long shot, but it happens, for not-so-common devices/setups.)

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