Entities may have duplicate policies

[tionebsalocin]

Describe the bug
Entities may have duplicate policies

To Reproduce
Steps to reproduce the behavior:
This can be reproduced in dev mode: vault server -dev

1. Create an entity and assign several times the same policy:

curl \
    --header "X-Vault-Token: ..." \
    --request POST \
    --data '{"metadata": {"organization": "hashicorp","team": "vault"}, "policies": ["policies-ro", "policies-ro", "policies-ro", "policies-ro", "policies-ro", "policies-ro"]}' \
    http://127.0.0.1:8200/v1/identity/entity

Output:
{"request_id":"...","lease_id":"","renewable":false,"lease_duration":0,"data":{"aliases":null,"id":"...","name":"entity_67584b4f"},"wrap_info":null,"warnings":null,"auth":null}
2. Display the entity info and notice that duplicate policies are assigned

curl \
    --header "X-Vault-Token: vaultdev" \
    http://127.0.0.1:8200/v1/identity/entity/name/entity_67584b4f | jq .

Output:

{
  "request_id": "...",
  "lease_id": "",
  "renewable": false,
  "lease_duration": 0,
  "data": {
    "aliases": [],
    "creation_time": "2021-02-05T16:19:54.987245149Z",
    "direct_group_ids": [],
    "disabled": false,
    "group_ids": [],
    "id": "...",
    "inherited_group_ids": [],
    "last_update_time": "2021-02-05T16:19:54.987245149Z",
    "merged_entity_ids": null,
    "metadata": {
      "organization": "hashicorp",
      "team": "vault"
    },
    "name": "entity_67584b4f",
    "namespace_id": "root",
    "policies": [
      "policies-ro",
      "policies-ro",
      "policies-ro",
      "policies-ro",
      "policies-ro",
      "policies-ro"
    ]
  },
  "wrap_info": null,
  "warnings": null,
  "auth": null
}

Expected behavior
Either remove duplicate during the entity creation or send a "duplicate policy" error

Environment:

• Vault Server Version (retrieve with vault status): 1.5.7
• Vault CLI Version (retrieve with vault version): 1.5.7
• Server Operating System/Architecture: CentOS 7 5.4.93-1.el7.x86_64

Additional context
As we use consul as backend this can lead to data exceeding the maximum size of the kv