is it possible to powder coat on top of something already powder coated ?

Yes.

i just had my new gen 3's powder coated. the shop insisted on media blasting the old powder coating of before redoing them, the came out great.

From everything I know (probably not enough) if you redo over powdercoat with another coat it will not come out properly. Powdercoat is processed by using a dry, plastic type, paint chip electrostaticly charged to "stick" to an oppositely charged part. One of the benefits to this type of process is as the part becomes covered, the charge is lessened on the high current edges and the chips are attracted to the balance of the part. This provides the opportunity to evenly coat the product. If the part is being recoated, the edges will have a thicker coat after every attempt. It works exactly the same on E-Coat also. My opinion would be to strip them first. Whoever you are considering having do them should be able to tell you the technical details. Let me know if I'm all washed up. Processes keep improving.

Many of the candy colors are coated with a silver base and then recoated with a translucent top coat.

The more you put on increases the probability of spidering.

The ticket is to have a good base at the beginning. Pin holing occasionally results if the temps are short cycled.

It can be done, but is not recommended as most here have pointed out. Blasting powder coat is a difficult task. The fastest way I've found is to buy a few quarts of PVC Primer and soak your parts in it for a few days. Small parts can be soaked in a gollon can. It's still a chore to get all of it off, but it's the best way for a professional job.

I had a set of V/C's done from a sample color. Ended up looking like nothing like the sample. They were reshot a couple days later and turned out just as high quality finish as the 1st coat. Looked perfect still 2 yrs later wehn I sold the boat. After doing a search on the net recently on powdercoating, it was mentioned many times by professional coaters that 2 coats are done for the reason pachangaphina mentioned. As long as the 1st coat is good.

im prepping my powder coated manifolds for instalation by blocking all the mating surfaces true with a hard sanding block and some 320 grit paper.

I don't usually powder over powder unless I'm doing a candy. I've done it, and it's held up, but it's usually been done over fresh powder. If you want to remove powder coat, use Berryman Chemdip parts cleaner. 24hrs in this stuff, and the coating falls off in sheets. It's a great carb cleaner too. This stuff is the real deal, when you open the can, it looks like a NJ. swamp inside!! You can feel the birth defects coming on!!
http://www.berrymanproducts.com/Default.aspx?tabid=146