grub2 update doubles crashkernel parameter every update

[Bartłomiej Żogała]

Although crashkernel parameter is already specified, update tries to add additional one, according to diff:
 │ -GRUB_CMDLINE_LINUX_DEFAULT="crashkernel=384M-:128M quiet splash log_buf_len=1M"
 │ -#noplymouth
 │ +GRUB_CMDLINE_LINUX_DEFAULT="crashkernel=384M-:128M quiet splash log_buf_len=1M crashkernel=384M-:128M"

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: grub-efi-amd64 2.02-2ubuntu8.8
ProcVersionSignature: Ubuntu 4.15.0-38.41-generic 4.15.18
Uname: Linux 4.15.0-38-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.9-0ubuntu7.4
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sat Nov 3 16:42:04 2018
InstallationDate: Installed on 2015-05-08 (1275 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=pl_PL.UTF-8
 SHELL=/bin/bash
SourcePackage: grub2
UpgradeStatus: Upgraded to bionic on 2018-08-26 (68 days ago)