Note the spell description doesn't say the spell is triggered by being reduced to 0 HP, it says, "You can cast this spell..."
But before it says that it says, "You fall unconscious...". Being unconscious after the first casting, you are unable to cast further spells, regardless of their casting time. If we read that statement as a rule and not flavor text.
Let's unpack that further:
If cast as a standard action, you are immediately reduced to -1 hit points, but stable, after casting the spell.
If your hit point total is negative, but not equal to or greater than your Constitution score, you're dying.
A dying character immediately falls unconscious and can take no actions.
So... "immediately after casting" you fall to -1 HP. There's no "immediate action" interruption mid-spell to cast it a second time... you don't fall to -1 HP until the casting is finished.
When you hit -1 HP, you "immediately fall unconscious".
An immediate action is not non-zero time... (from the Combat page above), but this spell explicitly says you can cast it as an immediate action in response to falling below 0 HP.
If we take the opening of the spell description as flavor text and not rules (the rest of the sentence supports such a reading), then I'm afraid I have to reverse my initial response. Technically, it seems you can fire this off twice in a row.
(I wouldn't allow it in my game, though.)