How many env: blocks per container?

While creating a Deployment earlier today, I faced a weird situation where a specific environment variable that was held as a Secret, was not being set. I tried deleting and recreating the secret, with no success. Mind you this was a long YAML with volumes, volumeMounts, ConfigMaps as enviroment variables, lots of lines. In the end the issue was pretty simple and I missed it because kubectl silently accepted the submitted YAML. I had two(!) env: blocks defined for the same container and somehow I missed that. It turns out, that when you do so, only the last one gets accepted, and whatever is defined in the previous, is not taken into account. To show this with an example:

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: box
  name: box
spec:
  replicas: 1
  selector:
    matchLabels:
      app: box
  template:
    metadata:
      labels:
        app: box
    spec:
      containers:
      - image: busybox
        name: busybox
        env:
        - name: FIRST_ENV
          value: "True"
        - name: ANOTHER_FIRST
          value: "True"
        command:
        - sleep
        - infinity
        env:
        - name: SECOND_ENV
          value: "True"

In the above example, when the Pod starts the container, only SECOND_ENV is set. FIRST_ENV and ANOTHER_FIRST are not.

I do not know whether this is a well known YAML fact or not, but it cost me some head-scratching and I hope it won’t cost you too.