Mongoose duplicated key

err.json

{
  "status": 400,
  "message": "insertDocument :: caused by :: 11000 E11000 duplicate key error index: morado.properties.$cert_1  dup key: { : \"predial2.pdf\" }",
  "data": {
    "code": 11000,
    "index": 0,
    "errmsg": "insertDocument :: caused by :: 11000 E11000 duplicate key error index: morado.properties.$cert_1  dup key: { : \"predial2.pdf\" }",
    "op": {
      "updatedAt": "2017-08-12T22:43:06.593Z",
      "createdAt": "2017-08-12T22:43:06.593Z",
      "title": "Casa remodelada en Chapultepec",
      "type": "598bf0f4b8fbcc16e6de79ff",
      "description": "Bonita casa amoblada y remodelada",
      "owner": "598a6794c7d97d3214d6abd3",
      "cert": "predial2.pdf",
      "_id": "598f847aee3221781cc3f665",
      "options": [],
      "status": 1,
      "access": 1,
      "contract": {
        "months": 2,
        "warranty": 1,
        "duration": 12
      },
      "rent": {
        "maintenance": 250,
        "cost": 3000
      },
      "address": {
        "line1": "Enrique Segoviano 8",
        "line2": "Lomas de Chapultepec",
        "zip": 1760,
        "city": "598a993021338ade511c6759",
        "region": "598a98eb21338ade511c65f1",
        "country": "598a98b121338ade511c6577"
      },
      "amenities": [],
      "files": [],
      "__v": 0
    }
  }
}