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JavaScript

var arr1 = [{
  "lastInteracted": "2016-03-31T11:13:09.000Z",
  "email": "concierge@inbound.com",
  "interactionCount": 2
}, {
  "lastInteracted": "2016-03-31T21:06:19.000Z",
  "email": "jbi@salesforce.com",
  "interactionCount": 1
}, {
  "lastInteracted": "2016-03-29T11:15:41.000Z",
  "email": "abc@insightsquared.com",
  "interactionCount": 1
}, {
  "lastInteracted": "2016-03-24T10:02:29.000Z",
  "email": "diana@hubspot.com",
  "interactionCount": 1
}]
var arr2 = [{
  "lastInteracted": "2016-03-31T11:13:09.000Z",
  "email": "concierge@inbound.com",
  "interactionCount": 2
}, {
  "lastInteracted": "2016-03-31T21:06:19.000Z",
  "email": "jbi@salesforce.com",
  "interactionCount": 4
}, {
  "lastInteracted": "2016-03-29T11:15:41.000Z",
  "email": "kstachowski@insightsquared.com",
  "interactionCount": 1
}, {
  "lastInteracted": "2016-03-24T10:02:29.000Z",
  "email": "hammer@hubspot.com",
  "interactionCount": 1
}, {
  "lastInteracted": "2016-03-24T10:02:29.000Z",
  "email": "life@hubspot.com",
  "interactionCount": 10
}, {
  "lastInteracted": "2016-03-24T10:02:29.000Z",
  "email": "mike@hubspot.com",
  "interactionCount": 18
}]

var ary = _.chain(arr1.concat(arr2))//use chain
  .groupBy(function(d) {
    return d.email;
  })//grouping by email
  .map(function(d) {
    var last = _.last(d);//take the last in the group
    var k = {
      email: last.email,
      lastInteracted: last.lastInteracted,
      interactionCount: _.reduce(d, function(memo, d1) {
        return memo + d1.interactionCount;//sum up interactionCount
      }, 0)
    };
    return k;
  }).value()

console.log(ary)