How to duplicate a specific object in a specific overlay from base with kustomize?

I have a base and two overlays: dev and prod, and all objects(Deployments, StatefulSets, Services etc.) defined in base layer are inherited in the two overlays. However, for some technical reason, I'd like to duplicate a specific Service "foo" in overlay prod to "foo-a", "foo-b", "foo-c", but keep it unchanged in overlay dev. Like the following:

.
├── base
│   ├── Service:foo
│   ├── Deployment:bar
│   ├── StatefulSet:baz
│   └── ...
└── overlays
    ├── dev
    │   ├── Service:foo      <-- base/Service:foo kept unchanged
    │   ├── Deployment:bar
    │   ├── StatefulSet:baz
    │   └── ...
    └── prod
        ├── Service:foo-a    <-- duplicated from base/Service:foo
        ├── Service:foo-b    <-- duplicated from base/Service:foo
        ├── Service:foo-c    <-- duplicated from base/Service:foo
        ├── Deployment:bar
        ├── StatefulSet:baz
        └── ...

Currently, the multibases solution is the closest solution, however, this solution duplicates all objects in a layer, but not a specific one.

I think I might use a script to achieve this, but I prefer the "kustomize" way. Any ideas? Thanks.

You can't do what you want with your existing layout; Kustomize is only able to include directories, not individual files, and there's no facility for "duplicating" a resource. If we move your Service manifest into a component and modify your overlays a bit, we can get closer.

I ended up with the following layout (I didn't include your example statefuleset because it wasn't relevant to the answer):

.
├── base
│   ├── deployment.yaml
│   └── kustomization.yaml
├── components
│   └── service
│       ├── kustomization.yaml
│       └── service.yaml
└── overlays
    ├── dev
    │   └── kustomization.yaml
    └── prod
        ├── kustomization.yaml
        ├── service-a
        │   └── kustomization.yaml
        ├── service-b
        │   └── kustomization.yaml
        └── service-c
            └── kustomization.yaml

base/kustomization.yaml looks like this:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- deployment.yaml

And deployment.yaml looks like:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: example
spec:
  template:
    spec:
      containers:
      - name: example
        image: docker.io/traefik/whoami:latest
        env:
        - name: WHOAMI_PORT_NUMBER
          value: "8080"
        ports:
        - containerPort: 8080
          name: http

Note that here and elsewhere I'm relying on kustomize to fill in labels and selectors.

components/service/kustomization.yaml looks like this (note the apiVersion):

apiVersion: kustomize.config.k8s.io/v1alpha1
kind: Component

resources:
- service.yaml

And service.yaml looks like this:

apiVersion: v1
kind: Service
metadata:
  name: example
spec:
  ports:
  - name: http
    port: 8080

Then we create overlays/dev/kustomization.yaml like this:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
commonLabels:
  app: example
  environment: dev
resources:
- ../../base

components:
- ../../components/service/

Running kustomize build overlays/dev gets us:

apiVersion: v1
kind: Service
metadata:
  labels:
    app: example
    environment: dev
  name: example
spec:
  ports:
  - name: http
    port: 8080
  selector:
    app: example
    environment: dev
---
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: example
    environment: dev
  name: example
spec:
  selector:
    matchLabels:
      app: example
      environment: dev
  template:
    metadata:
      labels:
        app: example
        environment: dev
    spec:
      containers:
      - env:
        - name: WHOAMI_PORT_NUMBER
          value: "8080"
        image: docker.io/traefik/whoami:latest
        name: example
        ports:
        - containerPort: 8080
          name: http

overlays/prod is a little more interesting. overlays/prod/kustomization.yaml looks like this:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
commonLabels:
  app: example
  environment: prod
resources:
- ../../base
- service-a
- service-b
- service-c

overlays/prod/service-a/kustomization.yaml looks like this:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
nameSuffix: -a

components:
- ../../../components/service/

service-b and service-c are similar; we just use a different nameSuffix. Running kustomize build overlays/prod results in a Deployment and three Services:

apiVersion: v1
kind: Service
metadata:
  labels:
    app: example
    environment: prod
  name: example-a
spec:
  ports:
  - name: http
    port: 8080
  selector:
    app: example
    environment: prod
---
apiVersion: v1
kind: Service
metadata:
  labels:
    app: example
    environment: prod
  name: example-b
spec:
  ports:
  - name: http
    port: 8080
  selector:
    app: example
    environment: prod
---
apiVersion: v1
kind: Service
metadata:
  labels:
    app: example
    environment: prod
  name: example-c
spec:
  ports:
  - name: http
    port: 8080
  selector:
    app: example
    environment: prod
---
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: example
    environment: prod
  name: example
spec:
  selector:
    matchLabels:
      app: example
      environment: prod
  template:
    metadata:
      labels:
        app: example
        environment: prod
    spec:
      containers:
      - env:
        - name: WHOAMI_PORT_NUMBER
          value: "8080"
        image: docker.io/traefik/whoami:latest
        name: example
        ports:
        - containerPort: 8080
          name: http

I think that's roughly what you were looking for.