command option is not being overridden when using multiple compose files with docker stack deploy

Description
I'm using multiple compose files for overriding service definitions for different environments, I noticed that when I try to override a service's command using another compose file it doesn't replace the original value but both values are used together.

Steps to reproduce the issue:

1. Create a compose file with a service and set the command option to any value
2. Create another compose file to override the command option in the first file
3. Deploy the service using docker stack deploy -c docker-compose.yml -c docker-compose.dev.yml stack
4. Check the service arguments using docker service inspect SERVICE_NAME -f '{{ .Spec.TaskTemplate.ContainerSpec.Args }}'

Example compose file:

version: '3.7'

services:

  service:
    image: busybox
    command: sh -c "while true; do echo $$HOSTNAME; sleep 2; done"

Another compose file to override the command option:

version: '3.7'

services:

  service:
    command: sh -c "while true; do echo $$HOME; sleep 2; done"

Describe the results you received:
Both command values are used together.

$ docker service inspect SERVICE_NAME -f '{{ .Spec.TaskTemplate.ContainerSpec.Args }}'
[sh -c while true; do echo $HOSTNAME; sleep 2; done sh -c while true; do echo $HOME; sleep 2; done]

Describe the results you expected:
We expect the command option to be overriden.

$ docker service inspect SERVICE_NAME -f '{{ .Spec.TaskTemplate.ContainerSpec.Args }}'
[sh -c while true; do echo $HOME; sleep 2; done]

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client:
 Version:           18.09.1-rc1
 API version:       1.39
 Go version:        go1.10.5
 Git commit:        bca0068
 Built:             Fri Dec  7 05:30:33 2018
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          18.09.1-rc1
  API version:      1.39 (minimum version 1.12)
  Go version:       go1.10.5
  Git commit:       bca0068
  Built:            Fri Dec  7 04:59:10 2018
  OS/Arch:          linux/amd64
  Experimental:     false

Output of docker info:

Containers: 3
 Running: 0
 Paused: 0
 Stopped: 3
Images: 121
Server Version: 18.09.1-rc1
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
Swarm: active
 NodeID: ysu1ke2vx275m5tzbh83pl4op
 Is Manager: true
 ClusterID: 7nuqacmpewluzeb2zhe8v3oeo
 Managers: 1
 Nodes: 3
 Default Address Pool: 10.0.0.0/8  
 SubnetSize: 24
 Orchestration:
  Task History Retention Limit: 5
 Raft:
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 10
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
  Force Rotate: 0
 Autolock Managers: false
 Root Rotation In Progress: false
 Node Address: 192.168.99.1
 Manager Addresses:
  192.168.99.1:2377
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: de1f167ab96338a9f5c2b17347abf84bdf1dd411
runc version: 10d38b660a77168360df3522881e2dc2be5056bd
init version: fec3683
Security Options:
 seccomp
  Profile: default
Kernel Version: 4.9.0-8-amd64
Operating System: Debian GNU/Linux 9 (stretch)
OSType: linux
Architecture: x86_64
CPUs: 4
Total Memory: 3.762GiB
Name: debian
ID: NW3S:FKHM:UGZW:ZXF7:IFXW:7D7O:TVBJ:WXNE:7T34:2SVL:JVIV:SWZO
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Username: mdawar
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false
Product License: Community Engine

WARNING: No swap limit support

Additional environment details (AWS, VirtualBox, physical, etc.):
Tested on Docker CE versions 18.09.1-rc1 and 18.06.1 on both Debian 9 and VirtualBox (boot2docker) using docker-machine.

Start with the docker stack deploy entry point and how it processes the two -c compose files, then trace merging of the service command option. Reproduce with the provided compose files and verify with docker service inspect that only the overriding command appears in the service arguments.