anaconda: avoid adding duplicated kernel entries

List kernel versions without duplicates, even when there are multiple
files related to the same kernel version.
Duplicated kernel versions here caused regenerating initramfs multiple
times and duplicated entries in xen.cfg.

QubesOS/qubes-issues#3624

anaconda/pyanaconda/packaging/__init__.py

@@ -829,7 +829,7 @@ class PackagePayload(Payload):
                 if fnmatch(f, "/boot/vmlinuz-*") or
                    fnmatch(f, "/boot/efi/EFI/%s/vmlinuz-*" % self.instclass.efi_dir)))
 
-        return sorted(files, key=functools.cmp_to_key(versionCmp))
+        return sorted(set(files), key=functools.cmp_to_key(versionCmp))
 
     @property
     def rpmMacros(self):