title: Combine arrays when grouping documents
body: <p>Documents in the database are in below format</p>
  
  <pre><code>{
    product: 'product1',
    state: 'state1',
    nondnd: [1, 2, 3],
    dnd: [4, 5],
    land: [],
    emails: ['a', 'b', 'c']
  },
  {
    product: 'product1',
    state: 'state1',
    nondnd: [9, 8, 2],
    dnd: [10, 7, 11],
    land: [2, 4, 6, 8],
    emails: ['d']
  },
  {
    product: 'product1',
    state: 'state2',
    nondnd: [9, 8, 2],
    dnd: [10, 7, 11],
    land: [1, 3],
    emails: ['e', 'g']
  }
  </code></pre>
  
  <p>I need to group the above documents on the basis of product &amp; state and get them in below format</p>
  
  <pre><code>{
    _id: {
      product: 'product1',
      state: 'state1'
    },
    nondnd: [1, 2, 3, 9, 8, 2],
    dnd: [4, 5, 10, 7, 11],
    land: [2, 4, 6, 8],
    emails: ['a', 'b', 'c', 'd']
  },
  {
    _id:{
      product: 'product1',
      state: 'state2'
    },
    nondnd: [2, 5, 8],
    dnd: [1, 4, 7],
    land: [1, 3],
    emails: ['e', 'g']
  }
  </code></pre>
  
  <p>I tried to unwind them individually and group them. But when I unwind them, same number is repeating. Please help me out</p>