.env overwrites other env file variables if existing

[mikoyats]

Description

Steps to reproduce the issue:

1. Create two env files

.env

FOO=OVERWRITE

test.env

FOO=ORIGINAL
BAR=ORIGINAL

2. Create docker-compose.yml

version: '3'

services:
  test1:
    container_name: test1
    image: node
    env_file: test.env
    command: env

3. Run docker-compose up

Describe the results you received:
Received the env variables:

test1  | NODE_VERSION=18.7.0
test1  | HOSTNAME=54379d4daed7
test1  | YARN_VERSION=1.22.19
test1  | HOME=/root
test1  | PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
test1  | BAR=ORIGINAL
test1  | FOO=OVERWRITE
test1  | PWD=/

Describe the results you expected:

test1  | NODE_VERSION=18.7.0
test1  | HOSTNAME=54379d4daed7
test1  | YARN_VERSION=1.22.19
test1  | HOME=/root
test1  | PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
test1  | BAR=ORIGINAL
test1  | FOO=ORIGINAL
test1  | PWD=/

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

Output of docker compose version:

Docker Compose version v2.6.1

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc., v0.8.2)
  compose: Docker Compose (Docker Inc., v2.6.1)
  extension: Manages Docker extensions (Docker Inc., v0.2.7)
  sbom: View the packaged-based Software Bill Of Materials (SBOM) for an image (Anchore Inc., 0.6.0)
  scan: Docker Scan (Docker Inc., v0.17.0)

Server:
 Containers: 52
  Running: 40
  Paused: 0
  Stopped: 12
 Images: 121
 Server Version: 20.10.17
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runtime.v1.linux runc io.containerd.runc.v2
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 10c12954828e7c7c9b6e0ea9b0c02b01407d3ae1
 runc version: v1.1.2-0-ga916309
 init version: de40ad0
 Security Options:
  seccomp
   Profile: default
  cgroupns
 Kernel Version: 5.10.104-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 10
 Total Memory: 15.64GiB
 Name: docker-desktop
 ID: P7XW:OVJL:ISLJ:FAOU:H3LF:T4VR:E5CP:UIR5:QKHA:ODYR:NDPA:WRHZ
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 HTTP Proxy: http.docker.internal:3128
 HTTPS Proxy: http.docker.internal:3128
 No Proxy: hubproxy.docker.internal
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  hubproxy.docker.internal:5000
  127.0.0.0/8
 Live Restore Enabled: false

Additional environment details: