In my experience, it will not bond very well, although it may seem to if you lay down a big bead of it that covers the old stuff. Generally, it's usually better to cut away old caulk -- of any kind -- with a razor blade before re-caulking.

What are you trying to do?

No.

Adding almost any kind of caulk/sealer/etc. over another, even the same type/kind/brand, is not a good practice.

Silicon glazes when cured, creating a smooth imprevious finish that resists moisture/dirt/etc.

So, if there is a problem with an existing area, remove all the old sealer, clean with alcohol, make sure all moisture and foreign materials are gone, and reapply new material.

Ok, I called Dow Corning and they said that it will adhere and create a water tight seal. I'll rough up the silicone where I add to it just in case and hope for the best.

I'm sealing an aquarium.

Well, good luck. If it starts leaking in a while, follow the advice about removing the old stuff.