What happened:
If you update a deployment with duplicate ports, there's no deterministic behaviour for which ports kubernetes will preserve in the deployment. It may remove all duplicate ports, keep the last one, or the first one. The behaviour gets even more unpredictable as the number of ports increase.

What you expected to happen:
Duplicate ports should be left untouched, exactly as they were specified in the applied YAML

How to reproduce it (as minimally and precisely as possible):

1. Apply this Deployment which contains duplicate ports. [Expect: created deployment contains all ports in the YAML. Actual: created deployment contains all ports]

2. Now, remove the port named 'five'. [Expect: all ports excluding 'five' should be present. Actual: a non-deterministic list of ports get deleted]

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-test
  namespace: sre
  labels:
    app: nginx-test
    env: staging
    owner: sre
spec:
  replicas: 2
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - name: one
          containerPort: 8000
        - name: two
          containerPort: 8000
        - name: three
          containerPort: 9000
        - name: four
          containerPort: 8000
        - name: five
          containerPort: 8000
        - name: six
          containerPort: 7000
        - name: seven
          containerPort: 7000
        resources:
          limits:
            cpu: 100m
            memory: 200Mi
          requests:
            cpu: 100m
            memory: 200Mi

Anything else we need to know?: n/a

Environment: GKE

• Kubernetes version (use kubectl version): 2 versions tested.

Client Version: version.Info{Major:"1", Minor:"17", GitVersion:"v1.17.4", GitCommit:"8d8aa39598534325ad77120c120a22b3a990b5ea", GitTreeState:"clean", BuildDate:"2020-03-12T23:40:44Z", GoVersion:"go1.14", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"17+", GitVersion:"v1.17.9-gke.600", GitCommit:"0bd89e105de2e1718e7418e73db3312050551a2b", GitTreeState:"clean", BuildDate:"2020-07-20T18:10:21Z", GoVersion:"go1.13.9b4", Compiler:"gc", Platform:"linux/amd64"}

Client Version: version.Info{Major:"1", Minor:"17", GitVersion:"v1.17.4", GitCommit:"8d8aa39598534325ad77120c120a22b3a990b5ea", GitTreeState:"clean", BuildDate:"2020-03-12T23:40:44Z", GoVersion:"go1.14", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"16+", GitVersion:"v1.16.11-gke.5", GitCommit:"baccd25d44f1a0d06ad3190eb508784efbb990a5", GitTreeState:"clean", BuildDate:"2020-06-25T22:55:26Z", GoVersion:"go1.13.9b4", Compiler:"gc", Platform:"linux/amd64"}

• Cloud provider or hardware configuration: Google Kubernetes Engine
• OS (e.g: cat /etc/os-release): n/a
• Kernel (e.g. uname -a): n/a
• Install tools: n/a
• Network plugin and version (if this is a network-related bug): n/a
• Others: n/a