As the title says, heat shimmer doesn’t work as written. Additional copies don’t reinforce the creature, just create a copy (essentially cutting the effect of the card to 1/3 of what is is supposed to be)
I just cast HS twice and it seemed to work correctly. What is it you tried it on?
I played my HS on the first node of the tour de Ravnica event.
First time was on a Nova Hellkite that was owned by my opponent.
Second time (same turn) was on a Nova Hellkite owned by me. The damage in the battle log seems to confirm that the NH was copied and reinforced the correct number of times.