Parsers - selecting last element in an array without specifying exact index?

[as-1]

hey, wondering if anyone has any decent ideas to solve this condundrum in CBN 

given the below json object to parse into additional.fields with keys being Unique instances of "match_key", creating repeated fields from duplicate keys, how do you conquer this in CBN? I've included a snippet of some python code I have created that gets the desired result 

{
    "data_objs": [
        {
            "match_key": "test3",
            "match_string": "string.com"
        },
        {
            "match_key": "test2",
            "match_string": "string.xyz"
        },
        {
            "match_key": "test1",
            "match_string": "string.tech"
        },
        {
            "match_key": "test1",
            "match_string": "string.tech"
        },
        {
            "match_key": "RuleName",
            "match_string": "string.tech"
        }
    ],
    "domain": "somewhere.com",
    "issue_org": "Let's Encrypt"
}

 
python code 

result_object_udm = {
    "metadata": {
    "event_timestamp": datetime.now().strftime("%Y-%m-%dT%H:%M:%S.%fZ"),
    "event_type": "GENERIC_EVENT",
    "product_name": "TEST_PRODUCT",
    },
    "additional": {
        "fields": [
        ]
    }
}
unique_matches = []
result_store = []
for rule in data1['matched_rules']:
    if rule['match_key'] not in unique_matches:
        unique_matches.append(rule['match_key'])
        result_object = {
                "key": rule['match_key'],
                "value": {
                    "list_value": {
                        "values": [
                            {
                                "string_value": rule['match_value']
                            }
                        ]
                    }
                }
            }
        result_store.append(result_object)
    else :
        result_store[-1]['value']['list_value']['values'].append({
            "string_value": rule['match_value']
        })
    

for result in result_store:
    result_object_udm['additional']['fields'].append(result)
# print(json.dumps(result_store, indent=4))

I've got as far as line 35 in CBN before becoming confused at how to track which keys have been seen before and thus need merging,  while I can confirm that a key is or is not unique im not sure how to access the last element of an array to update the list_value.values array with the corresponding value

any ideas of alternative methods also appreciated