Rates of chemical reaction a clock reaction(what effect does doubling the concentration of I- have on the rate of the reaction?

Unless you know the order of reaction with respect to I-, or experimental results where the concentration of I- was varied keeping the concentration of the other reactants constant, you cannot deduce what will happen to the rate of reaction.

The effect on the reaction rate of varying the reactants is the whole point of the experiment. You have to do it. It is a wonderful reaction for this purpose. Mix the reagents, start timing, and POW, it turns purple/black at the end. There is no decision on your part to determine the completion -- no wondering if the color is dark enough, if it lasts long enough, etc. The only source of error is whether you are alert enough to stop timing at the INSTANT the color changes.
Try it, you'll like it! It is a spectacular reaction!