An Array of Challenges #1: Alternating Arrays
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ḣ2ṁ⁼


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How it works

ḣ2ṁ⁼  Main link. Argument: A (array)

ḣ2    Head 2; truncate A after its second element. If A has two or less elements,
      this returns A itself.
  ṁ   Mold; cyclically repeat the elements of the previous result to create an
      array that has the same shape/length as A.
   ⁼  Test the result for equality with A.