Duplicate consumers cause a JSON unmarshal error

[mheap]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

The response cannot be unmarshalled

2024-01-12T12:11:22Z	error	Failed parsing resource errors	{"url": "https://192.168.194.4:8444", "update_strategy": "InMemory", "error": "could not unmarshal config error: json: cannot unmarshal object into Go struct field ConfigError.flattened_errors of type []sendconfig.FlatEntityError"}
2024-01-12T12:11:22Z	error	dataplane-synchronizer	Could not update kong admin	{"error": "performing update for https://192.168.194.4:8444 failed: failed posting new config to /config: got status code 400"}

Expected Behavior

I see the actual error message in the logs + an event is raised

Steps To Reproduce

Deploy `kong/ingress` with defaults


echo "apiVersion: configuration.konghq.com/v1
kind: KongConsumer
metadata:
  name: alice1
  annotations:
    kubernetes.io/ingress.class: kong
username: alice
---
apiVersion: configuration.konghq.com/v1
kind: KongConsumer
metadata:
  name: alice2
  annotations:
    kubernetes.io/ingress.class: kong
username: alice" | kubectl apply -f -

See the logs:

kubectl logs -n kong kong-controller-....

### Kong Ingress Controller version

```shell
kong/kubernetes-ingress-controller@sha256:64a6927d9e00336dccd70fc670f8c46b41d737d52bf4c65f66be33ed1deae44a (KIC 3.0)

Kubernetes version

No response

Anything else?

No response

[pmalek]

With changes from #5763 (which will ship with KIC 3.2 and will be included in nightlies starting 29-03-2024) the log that we get from the attached manifest look like this:

2024-03-28T15:17:12+01:00	error	dataplane-synchronizer	Could not update kong admin	{"error": "performing update for https://10.244.0.11:8444 failed: failed posting new config to /config: got status code 400"}
2024-03-28T15:17:15+01:00	error	recording a Warning event for object	{"name": "alice2", "namespace": "default", "kind": "KongConsumer", "apiVersion": "configuration.konghq.com/v1", "reason": "KongConfigurationApplyFailed", "message": "invalid consumer:: uniqueness violation: 'consumers' entity with username set to 'alice' already declared", "error": "object failed to apply"}

When running with Kong kong/kong-gateway:3.6.

@mheap feel free to reopen this if you encounter this again.