1var array = [1, 2, 2, 3, 3, 4, 5, 6, 2, 3, 7, 8, 5, 22, 1, 2, 511, 12, 50, 22];
2
3console.log([...new Set(
4  array.filter((value, index, self) => self.indexOf(value) !== index))]
5);

1var input = [1, 2, 3, 1, 3, 1];
2
3var duplicates = input.reduce(function(acc, el, i, arr) {
4  if (arr.indexOf(el) !== i && acc.indexOf(el) < 0) acc.push(el); return acc;
5}, []);
6
7document.write(duplicates); // = 1,3 (actual array == [1, 3])

1const names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']
2
3const count = names =>
4  names.reduce((a, b) => ({ ...a,
5    [b]: (a[b] || 0) + 1
6  }), {}) // don't forget to initialize the accumulator
7
8const duplicates = dict =>
9  Object.keys(dict).filter((a) => dict[a] > 1)
10
11console.log(count(names)) // { Mike: 1, Matt: 1, Nancy: 2, Adam: 1, Jenny: 1, Carl: 1 }
12console.log(duplicates(count(names))) // [ 'Nancy' ]

1const arr = ["q", "w", "w", "e", "i", "u", "r"]
2arr.reduce((acc, cur) => { 
3  if(acc[cur]) {
4    acc.duplicates.push(cur)
5  } else {
6    acc[cur] = true //anything could go here
7  }
8}, { duplicates: [] })

1let a = [1, 2, 3, 4, 2, 2, 4, 1, 5, 6]
2let b = [...new Set(a.sort().filter((o, i) => o !== undefined && a[i + 1] !== undefined && o === a[i + 1]))]
3
4// b is now [1, 2, 4]

1var names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']
2
3var uniq = names
4  .map((name) => {
5    return {
6      count: 1,
7      name: name
8    }
9  })
10  .reduce((a, b) => {
11    a[b.name] = (a[b.name] || 0) + b.count
12    return a
13  }, {})
14
15var duplicates = Object.keys(uniq).filter((a) => uniq[a] > 1)
16
17console.log(duplicates) // [ 'Nancy' ]