JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = [{
    "lastName": "Seymour",
    "gender": "Female",
    "patientID": 18134,
    "firstName": "Stephani",
    "age": "111Y"
  },
  {
    "lastName": "Seymour",
    "gender": "Female",
    "patientID": 18134,
    "firstName": "Stephanie",
    "age": "111Y"
  }];
    var hash = (function() {
    var keys = {};
    return {
        contains: function(key) {
            return keys[key] === true;
        },
        add: function(key) {
            if (keys[key] !== true)
            {
                keys[key] = true;
            }
        }
    };
})();

var key = null;
for (var i = 0; i < arr.length; i++)
{
    key = arr[i].firstName + ";" + arr[i].lastName;
    if (!hash.contains(key))
    {
        hash.add(key);
    }
}