Overwriting Hosted Feature Service via REST

A quick skills background... Not a full time programmer, proficient enough in Python to get by but VERY new to REST API scripting. So forgive me if there is an easy answer, I am missing something simple or a different approach is warranted.

I am trying to update an existing hosted feature service by using the 'Publish Item' operation using the overwrite=true parameter. (API Reference: ArcGIS REST API - Publish Item).

The following function (minus overwrite param) works perfectly to publish the feature service from an existing geojson item I have already added to Portal. When I add the overwrite parameter, I get the message that a service with that name already exists. Seems like it checks the service name before recognizing the overwrite parameter. Service is not delete protected. Seems simple and straightforward adding of a parameter to the REST call. Resulting URLs from the original publishing POST call and the update call are the same except for '&overwrite=true' is added to the URL.

What am I missing?

Response Message: "Publish error for item 'CMT_Test3.geojson'. Service name 'CMT_Test3' already exists"

def PublishService(Short, itemID, username, token, itemName, publishParams):
    publishURL =   'http://{}/portal/sharing/rest/content/users/{}/publish'.format(Short, username)
    query_dict = {
        'itemID': itemID,
        'filetype': 'GeoJson',
        'f': 'json',
        'token': token,
        'overwrite': 'true',
        'publishParameters':publishParams}

    query_dict['publishParameters'] = json.dumps(query_dict['publishParameters'], sort_keys=False)
    request = requests.post(publishURL, data=query_dict)
    return request.json()