Find Duplicate Array within Array

Question

Given an array of arrays, what would be the efficient way of identifying the duplicate item?

var array = [
  [
    11.31866455078125,
    44.53836644772605
  ],
  [                     // <-- Here's the duplicate
    11.31866455078125,
    44.53836644772605
  ],
  [
    11.371536254882812,
    44.53836644772605
  ],
  [
    11.371536254882812,
    44.50140292110874
  ]
]

I've been working on this with lodash as an accepted dependency, and I get how to just return the "unique" list using _.uniqWith and _.isEqual:

_.uniqWith(array,_.isEqual)

With would give the "unique" version of the list:

[ 
    [ 11.31866455078125,  44.53836644772605 ],
    [ 11.371536254882812, 44.53836644772605 ],
    [ 11.371536254882812, 44.50140292110874 ]
]

But rather than just reporting the unique elements, I need just the element that is duplicated, and ideally the index of the first occurrence.

Is this actually covered in the lodash library by some combination of methods that I'm missing? Or am I just going to have to live with writing loops to compare elements.

Probably just overtired on this, so fresh eyes on the problem would be welcome.

Trying not to rewrite functions if there are library methods that suit, so I basically am stuck with:

1. Returning just the duplicate or at least the comparison difference with the "unique list".

2. Basically identifying the "index of" an array within an array. Though I suppose that can be a filter reduction with _.isEqual once the duplicate item is identified.

Trying also to avoid creating an object Hash/Map and counting the occurrences of keys here as well, or at least not as a separate object, and as something that can be done functionally "in-line".

Answer 1

Lodash gives a lot of useful functions to achieve finding the first duplicate index.
Using the _.findIndex() and _.isEqual() the following code will find the first duplicate index:

var duplicateIndex = _.findIndex(array, function(value, index, collection) {
  var equal = _.isEqual.bind(undefined, value);
  return _.findIndex(collection.slice(0, index), equal) !== -1;
});

or a bit faster but more verbose:

var duplicateIndex = _.findIndex(array, function(value, index, collection) {
  var equal = _.isEqual.bind(undefined, value);
  return _.findIndex(collection, function(val, ind) {
     return ind < index && equal(val);
  }) !== -1;
});

Notice that if no duplicate exists, -1 will be returned.
In a few words the algorithm iterates through array and looks back if the current element does not exist already. If it does, just return the current iteration index.
Please check the working demo.