E11000 duplicate key error collection in spring data mongo when using custom String as a primary key

I want to use a custom String code as a primary key in my collection and ensure that Mongo Repository save updates/upserts the document based on code. But when a 2nd entry is added with same code, it throws E11000 duplicate key error collection. PFB my code snippet :

public class WebCategoryTemp {

@Id
private String code;
@Indexed
private Long timestamp;
private String name;
private String parent;
private Map<String,String> attributes;
private List<String> brands;
private List<String> categories;
private List<String> optionCodes;

}

Repository:

public interface WebCategoryTempRepository extends 
MongoRepository<WebCategoryTemp, String> {
}

I am getting the below exception while adding another entry with same code :

org.springframework.dao.DuplicateKeyException: E11000 duplicate key error 
  collection: AQUAMAN.webCategoryTemp index: _id_ dup key: { _id: 
  "GlobalTrends" }; nested exception is com.mongodb.MongoWriteException: 
  E11000 duplicate key error collection: AQUAMAN.webCategoryTemp index: _id_ 
  dup key: { _id: "GlobalTrends" }

Sample Mongo Document:

{
    "_id" : "GlobalTrends",
    "timestamp" : NumberLong("1570439407216"),
    "name" : "GlobalTrendss",
    "parent" : "83",
    "attributes" : {
        "categoryLevel" : "4",
        "returnWindow" : "35",
        "isReturnable" : "true",
        "maximumQuantity" : "2",
        "categoryToggle" : "toggleCode",
        "catalogId" : "rilfnlProductCatalog"
    },
    "brands" : [ ],
    "categories" : [
        "830316001"
    ],
    "version" : NumberLong(0)
}