Fix Uniqueness check by there is relationship to the same object but under 2 relationships keys

[ecdatadog]

Spec : https://jsonapi.org/format/1.1/#document-resource-object-relationships
Allow some json+api document that have relationships to a same object but under differents relation key :

{
    "data": {
        "id": "1",
        "type": "articles",
        "attributes": {
            "title": "A"
        },
        "relationships": {
            "author": {
                "data": {
                    "id": "1",
                    "type": "author"
                }
            },
            "contact": {
                "data": {
                    "id": "1",
                    "type": "author"
                }
            },
            "comments": {
                "data": [
                    {
                        "id": "1",
                        "type": "comments"
                    }
                ]
            }
        }
    }
}