Core 2 Duo is an Intel trademark for their dual-core processors. If you refer strictly to Intel processors, the newer Core 2 Duo processors are faster than their older dual-core (Pentium D) processors. AMD also manufacture dual-core processors that can compete with Intel's offerings.
how many of these have we had in the last few weeks, i've answered a few myself, why are they popping up now, you can still buy c2d's for a while longer, but anything from the gerneration before that I doubt is out there being sold?
Core2Duo is a series in the Intel Core2 family. The 'Duo' is the dual(2) core series of Core2. (Just like how in Core2Quad the 'Quad' is the quad(4) core version of the Core2d series)
Dual core means that the CPU has 2 physical cores. Think of it like having 2 processors in 1.
So as you can see the difference between the 2 is that on is a title and one is an attribute. Core2Duo is in fact a dual core itself.