"Supposing" itself already introduces a conditional or hypothetical scenario. It means to assume or consider something as true for the sake of an argument or discussion. Adding "if" after "supposing" is redundant and grammatically incorrect because "supposing" already conveys the idea of a condition or a hypothetical situation.

So, when you say "Supposing if it rains," it's like saying "Assuming if it rains," which doesn't make sense because "assuming" or "supposing" already conveys the idea of taking a condition into account.

The correct sentence should be: "Supposing it rains, what shall we do?" In this corrected sentence, "Supposing" is used correctly to introduce a hypothetical scenario or condition without the redundant "if."