@Q:
RETURN UNIQUE(
  UNION(
      [ <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span> ],
      [ <span class="hljs-number">1</span>, <span class="hljs-number">2</span> ]
  )
)

@R
[
  [
    <span class="hljs-number">3</span>,
    <span class="hljs-number">2</span>,
    <span class="hljs-number">1</span>
  ]
]