post request allows to send data which already exists

Request Post
URI:
http://localhost:8080/restconf/17/data

Body:
{
"cars" : {
"car-entry": [

{ "year": "54688", "model": "string", "id": "some id", "category": "strhing", "manufacturer": "string" }

]
}
}

next with same uri and body
{
"cars" : {
"car-entry": [

{ "year": "1111", "model": "diff", "id": "some id", "category": "diff", "manufacturer": "diff" }

]
}
}

should not allow. it should respond with data already exist.