Validation is missing for namespaces and matchExpression used for namespaceSelector and serviceSelector as duplicate entries are allowed.
matchLabel accepts duplicate entries in YAML but silently drops when ip addresspool is created.

4.13

Always

1. Install metallb operator and create IP addresspool with YAML below:
apiVersion: metallb.io/v1beta1
kind: IPAddressPool
metadata:
  name: ip-addresspool-l3a
  labels:
    zone: east
  namespace: metallb-system
spec:
  addresses:
    - 10.10.1.1-10.10.1.10
  autoAssign: true
  avoidBuggyIPs: true
  serviceAllocation:
    priority: 50
    namespaces:
      - test
    namespaceSelectors:
      - matchLabels:
          team: operations
      - matchExpressions:
          - key: region
            operator: In
            values: 
              - 'na'
              - 'na'
 2. Another YAML
--
apiVersion: metallb.io/v1beta1
kind: IPAddressPool
metadata:
  name: ip-addresspool-l3a
  labels:
    zone: east
  namespace: metallb-system
spec:
  addresses:
    - 10.10.1.1-10.10.1.10
  autoAssign: true
  avoidBuggyIPs: true
  serviceAllocation:
    priority: 50
    namespaces:
      - test
      - test
    namespaceSelectors:
      - matchLabels:
          team: operations
      - matchExpressions:
          - key: region
            operator: In
            values: 
              - 'na'

IP address pool is created
--
oc get ipaddresspool ip-addresspool-l3a -n metallb-system -oyaml
apiVersion: metallb.io/v1beta1
kind: IPAddressPool
metadata:
  creationTimestamp: "2023-02-22T00:27:33Z"
  generation: 1
  labels:
    zone: east
  name: ip-addresspool-l3a
  namespace: metallb-system
  resourceVersion: "257675"
  uid: 998dc5cf-a2b4-48e0-b5f1-808d5d84edf0
spec:
  addresses:
  - 10.10.1.1-10.10.1.10
  autoAssign: true
  avoidBuggyIPs: true
  serviceAllocation:
    namespaceSelectors:
    - matchLabels:
        team: operations
    - matchExpressions:
      - key: region
        operator: In
        values:
        - na
        - na
    namespaces:
    - test
    priority: 50

 oc get ipaddresspool ip-addresspool-l3a -n metallb-system -oyaml
apiVersion: metallb.io/v1beta1
kind: IPAddressPool
metadata:
  creationTimestamp: "2023-02-22T00:42:50Z"
  generation: 1
  labels:
    zone: east
  name: ip-addresspool-l3a
  namespace: metallb-system
  resourceVersion: "263468"
  uid: a105edd2-9910-4163-a25a-5591177d7951
spec:
  addresses:
  - 10.10.1.1-10.10.1.10
  autoAssign: true
  avoidBuggyIPs: true
  serviceAllocation:
    namespaceSelectors:
    - matchLabels:
        team: operations
    - matchExpressions:
      - key: region
        operator: In
        values:
        - na
    namespaces:
    - test
    - test
    priority: 50

Warning message and ip address pool should not be created.