Got very interesting code in the answers. I would like to mention that I also could achieve the result using the lodash method .merge().

const result = _.merge(listOfQuestions, listOfAnswers)

const listOfQuestions = [{question1:{important: true}}, {question2:{important: false}}]
const listOfAnswers = [{question1:{answer: false}}, {question2:{answer: true}}]

const result = _.merge(listOfQuestions, listOfAnswers)
console.log(result)

<script src="https://cdn.jsdelivr.net/npm/lodash@4.17.10/lodash.min.js"></script>

Here is one option:-

var array = [{
  name: "foo1",
  value: "val1"
}, {
  name: "foo1",
  value: ["val2", "val3"]
}, {
  name: "foo2",
  value: "val4"
}];

var output = [];

array.forEach(function(item) {
  var existing = output.filter(function(v, i) {
    return v.name == item.name;
  });
  if (existing.length) {
    var existingIndex = output.indexOf(existing[0]);
    output[existingIndex].value = output[existingIndex].value.concat(item.value);
  } else {
    if (typeof item.value == 'string')
      item.value = [item.value];
    output.push(item);
  }
});

console.dir(output);

Use a lodash chain to concat the arrays, group similar objects, and then merge each group to a single object:

var arr1 = [{ a: "a", 1: 1, 2: 2 }, { a: "b", 1: 1, 2: 3 }];
var arr2 = [{ a: "a", 3: 123 }, { a: "b", 3: 4411 }];

var result = _(arr1)
  .concat(arr2) // concat the 2nd array
  .groupBy('a') // group by the identical key
  .map(_.spread(_.curry(_.merge, {}))) // left currey merge to to create a new empty object, and spread the group as parameters
  .value();
  
console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.4/lodash.min.js"></script>

With ES6 you can use Array#reduce to collect the similar objects in a Map, then get the Map#values iterator, and use the spread syntax to convert to an array:

const arr1 = [{ a: "a", 1: 1, 2: 2 }, { a: "b", 1: 1, 2: 3 }];
const arr2 = [{ a: "a", 3: 123 }, { a: "b", 3: 4411 }];

const result = [...arr1.concat(arr2) // concat the arrays
  .reduce((m, o) => m.set(o.a, Object.assign(m.get(o.a) || {}, o)), // use a map to collect similar objects
  new Map()
).values()]; // get the values iterator of the map, and spread into a new array
  
console.log(result);

Here the solution using lodash and uniqWith function: JSFiddle.

Here's the code:

function merge(array) {
  return _.uniqWith(array, compareAndMerge)
}

function compareAndMerge(first, second) {
    if (first.field === second.field) {
        first.values = second.values = [].concat(first.values, second.values)
        return true
    }
    return false
}


var data = [{
  field: 'Currency',
  operator: 'IN',
  values: ['usd']
}, {
  field: 'Currency',
  operator: 'IN',
  values: ['gbp']
}, {
  field: 'Amount',
  operator: 'IN',
  values: [2]
},
{
  field: 'Amount',
  operator: 'IN',
  values: [3]
}]

console.log(merge(data))

Lodash.uniqWidth function wants an array and a comparator. In case of equal fields we edit values of the two compared elements assigning the concatenation of the two values arrays.

Something more: it's a transgression to edit objects inside the comparator, but I think that it could run safely.

You can iterate over the object keys and create a new object out of their aggregation.

You can use ES6 spread operator (...) which allows us to quickly copy all or part of an existing array or object into another array or object.

const cat1 = { med1: { a: 10, b: 12 }, med2: { c: 14, d: 16 } };
const cat2 = { med1: { e: 18, f: 20 }, med2: { g: 22, h: 24 } };
let resultObject = {};
Object.keys(cat1).map(key => { // iterate over the keys
  resultObject = {
    ...resultObject,
    [key]: {...cat1[key], ...cat2[key]} // merge two objects 
  }
  return;
});

console.log(resultObject);

You can use reduce for that and iterate over the object keys of each object:

const res = array.reduce((a, b) => {
    Object.keys(b).forEach(k => {
        a[k] = (a[k] || 0) + b[k];
    });
    return a;
}, Object.create(null));

console.log(res);

<script>
const array = [
    {
        RT: 0.3396966806905289,
        Real: 0.03721461991442617,
        fired: 0.26138293747172237,
        'Real Madrid': 0.15767570563383954,
        Madrid: 0.2771992640023346,
        Zidane: 0.3218878635800967,
        'Florentino Perez': 0.31535141126767907,
        leave: 0.1632670522868711,
        manager: 0.12690962022792524,
        tancredipalmeri: 0.3218878635800967,
        Perez: 0.4543985280046692,
        Florentino: 0.44189852800466917
    },
    {
        Cannavaro: 0.2709492640023346,
        Ashley: 0.2771992640023346,
        'Ashley Cole': 0.15767570563383954,
        Casillas: 0.3896966806905289,
        Ronaldo: 0.22094926400233458,
        Zidane: 0.22094926400233458,
        Messi: 0.22094926400233458,
        Lampard: 0.22094926400233458,
        Iniesta: 0.22094926400233458,
        Terry: 0.2771992640023346,
        Henry: 0.19890866571173632,
        Cole: 0.22094926400233458,
        Ivanovic: 0.3218878635800967
    },
    {
        RT: 0.3396966806905289,
        effleurer: 0.22094926400233458,
        'n\'a': 0.2709492640023346,
        top: 0.13189246756926315,
        Zidane: 0.2709492640023346,
        entraineur: 0.22094926400233458,
        monde: 0.22094926400233458,
        au: 0.2271992640023346,
        'au monde': 0.20434237230050623,
        Mvssi10: 0.3218878635800967,
        JAMAIS: 0.2709492640023346
    },
    {
        RT: 0.3396966806905289,
        Real: 0.10790900513119943,
        FrenchRMCF: 0.3218878635800967,
        en: 0.2271992640023346,
        histoire: 0.2709492640023346,
        coach: 0.2271992640023346,
        au: 0.2271992640023346,
        Zidane: 0.22094926400233458,
        'en tant': 0.15767570563383954,
        tant: 0.22094926400233458
    }
]
</script>