I was wondering if there were any experienced carpenters in the house. I'm putting down a new hardwood floor in my kitchen/dining room. The rooms are on the second floor and already have a layer of 3/4" plywood and 3/4" hardwood in the dining room. The kitchen has two layers of 3/4" plywood.

Do I need to rip up any of this flooring to put down my new 3/4" wood floor? I don't know if it's safe to just put down the new stuff over the old. My second floor has support walls from the bathroom and bedroom below, and has 2x8 floor joists. I'm trying not to have to rip up the old floor because it runs into my living room and don't want that big of a mess. However, if it will put a structural strain and is unsafe I'll have no choice.

So what do you guys think?

I'd rip up the old hard wood floor and see how the oringal sub floor looks.  If there was any squeeks i'd toss in some screws and then just install the new hardwood on top.

Laying a new hardwood floor over an old hardwood floor is not a good idea. You could easily create expansion/contraction conditions that will ruin the new floor.

You should pull it up. If the top layer of 3/4" plywood in the other room is in good shape and fastened down extremely well, then I would just leave it and fill in the height difference in the room that you ripped the old hardwood floor out of.

The best option is to go down to the original subfloor where you plan to install the new floor, and re-nail all of the subfloor to tighten it up before installing the new stuff.