On what basis is Genesis 1 said to be Hebrew poetry? I've heard several knowledgeable people state that Genesis 1 is using Hebrew poetic language, but I recently ran across some counter arguments against the view that Genesis 1 contains poetry in the original language.* 

What basis do interpreters use to support their idea that Genesis 1 is using poetic language? I am Catholic and thus am interested in anything consistent with that Tradition.

*Those arguments included the idea that Hebrew poetry appears to repeat ideas twice in the same statement; however it then only then addressed other parts of Genesis, as if it were assumed that if Gen 1 is poetry, the whole book must be.
Genesis 1 is constrained writing, and most poetry is a kind of constrained writing. My guess is that this is what has led a lot of people to say that Genesis 1 is a kind of poetry. But not all constrained writing is poetry, and Genesis 1 is very dissimilar from the typical Hebrew poetry of the rest of the Bible (ie., the Psalms, most of the prophets etc.)

To explain how Genesis 1 is constrained writing, I'll quote this list from John Dickson explaining all the ways in which the number seven and multiples of seven are used in the chapter:


  
  The first sentence has 7 Hebrew words
  The second sentence has 14 Hebrew words
  The words for 'earth' and 'heaven' each occur 21 times
  God is mentioned 35 times
  'and it was so' occurs 7 times
  'God saw that it was good' occurs 7 times
  The chapter is structured around 7 days
  


The number seven symbolises wholeness and perfection, the very quality of the completed creation. Genesis 1 reinforces the goodness of the creation both through what it teaches, but also how it teaches, using these subtle (and invisible in English translations) constraints on its writing.