Restkit NSInternalInconsistencyException Unable to add mapping for keyPath

Question

Fyi, I'm not too familiar with Restkit's API. I prefer other means of deserializing json into domain objects; however, I've inherited a project and this is the quickest solution to fix a bug.

I'm trying to map "assignments" in the below json to "doctorAssignments" which is a 1-Many relationship in Core Data.

example json:

{
    "id": 43,
    "time_zone": "Mountain Time (US & Canada)",
    "accepting_patients": true,
    "auto_set_context_id": 44,
    "audio_notifications": false,
    "assignments": [
        {
            "id": 14,
            "active_since": "2015-06-17T23:26:03Z",
            "doctor": {
                "id": 44,
                "name": "Darth Vader PsyD",
                "title": "Dr.",
                "bio": "",
                "type": "Doctor",
                "profile_image_url": "https://cirrusmd-staging.s3.amazonaws.com/uploads/doctor/profile_image/43/darth.png?X-Amz-Expires=600&X-Amz-Date=20150618T163351Z&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAJGWZJVKLZCGMXJ2Q/20150618/us-east-1/s3/aws4_request&X-Amz-SignedHeaders=host&X-Amz-Signature=b0bc0163f4cf8c3c73572198c596565b0012a33331936baf4b14b9a255c0870f",
                "credential_id": 214
            }
        }
    ],
    "practice": {
        "id": 2,
        "name": "Test Practice",
        "address": "",
        "phone": "",
        "fax": "",
        "created_at": "2013-06-11T17:49:48Z",
        "updated_at": "2013-06-11T17:49:48Z",
        "email": "",
        "city": "",
        "state": "",
        "zip": ""
    }
}

Mapping:

    + (RKEntityMapping *)responseMapping {
    if (_responseMapping == nil) {
        _responseMapping = [RKEntityMapping mappingForEntityForName:[CMDUser entityName] inManagedObjectStore:[NSManagedObject managedObjectStore]];
        _responseMapping.identificationAttributes = @[ CMDUserAttributes.userID, CMDUserAttributes.type ];

        [_responseMapping addAttributeMappingsFromDictionary:@{
            @"id" : CMDUserAttributes.userID,
            @"type" : CMDUserAttributes.type,
            @"profile_image_url" : CMDUserAttributes.profilePic,
            @"credential_id" : CMDUserAttributes.credentialID,
            @"name" : CMDUserAttributes.name,
            @"last_name" : CMDUserAttributes.lastName,
            @"first_name" : CMDUserAttributes.firstName,
            @"allergies" : CMDUserAttributes.allergies,
            @"medications" : CMDUserAttributes.medications,
            @"history" : CMDUserAttributes.history,
            @"email" : CMDUserAttributes.email,
            @"location" : CMDUserAttributes.location,
            @"zipcode" : CMDUserAttributes.zipcode,
            @"gender" : CMDUserAttributes.gender,
            @"dob" : CMDUserAttributes.dob,
            @"relationship.state" : CMDUserAttributes.relationshipState,
            @"relationship.status" : CMDUserAttributes.relationshipStatus,
            @"relationship.onboarded_at" : CMDUserAttributes.onboardedAt,
            @"auto_set_context_id" : CMDUserAttributes.autoSetContextID,
            @"phone" : @"phone",
        }];
    }
    // taking this line out removes the exception, but then the json isn't mapped
    [_responseMapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:@"assignments" toKeyPath:@"doctorAssignments" withMapping:[CMDDoctorAssignment mapping]]];

    return _responseMapping;
}

On launch I get this error: Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Unable to add mapping for keyPath doctorAssignments, one already exists...'

Note: I've omitted some fields in the json. In the mapping, I do NOT have

@"assignments": @"doctorAssignments".