Remove duplicates from an array (to be used in Array.reduce()). If array.length may be 1, an emty array needs to be supplied as an initial value.

makeUnique.js

function makeUnique(prev, current, index) { 
  return (index === 1 
    ? [prev].concat(prev !== current ? current : []) // handle when first two items are identical
    : (prev.indexOf(current) > -1 
      ? prev
      : (prev.push(current), prev)))
}