there is nothing wrong with your computer. this is the way they are now. there is some sort of auxilary or subset to each core and thats why you have the double in your activity monitor. how it really works...? i dont know.
im sure someone here more technical can explain whats going on. but no, there is nothing wrong with your computer.
The new quad core, which are Intel i7, have hyperthreading, so each core has two virtual cores. And an 8-core isn't actually a single 8-core prcoessor, it's 2 quad core processors.
