Is double R-13 as good as single R-30 in a cantilever?

No, it won't be as good. But the difference between R 26 and R 30 is hardly worth mentioning. Yes, it would be better to replace the insulation and check for voids and missing insulation if you want to. It seems to me to be a good bit of expense for a little bit of improvement in performance but that is your decision.

There are several factors to consider. First is just what are you trying to accomplish, what are you trying to insulate, how much space is available and what is your primary consideration: cost or performance.

You say it is in a cantilever. Somehow I imagine that you are trying to resolve the issue of a cold floor. In my experience insulation does not resolve this issue. The problem is more related to heat radiating downward and air infiltration than heat losses due to convection. A better vapor barrier and another ground source of heat will work better than the insulation.

If it is an attic or overhead space that you are concerned with you still need to be careful not to over insulate. You must have 2" of space behind your insulation. For the 9 1/2 " of R 30 you have to have a minimum of 2 x 12 timber. The R 13 is only going to need 7" of space (3 1/2" x 2) and can be put into a 2 x 10" space.

Another consideration is that the R 13 is just R 11 insulation with a foil face. This works for the first layer but for the second layer you might want to put in unfaced insulation and so it would be R-11 that you would be installing. But still the difference between a R24 total and a R 30 may still not be worth the investment in labor to remove and dispose of the first layer. You can check for foam insulation in only one bay without removing the whole lot if necessary.