Of course you can get dual core i5's. They could quite easily fix up the 21.5" iMac's with dual core i processors to reduce cost and heat, especially considering that the 21.5" models arent particularly high end.
And an iMac isnt a desktop as such, as they use mobile GPUs.
Still though, I'd expect this to be fake.
No, ever since Sandy Bridge was introduced, dual core i5 are a thing of the past.
Why would they put an "old" i5 when Sandy Bridge is so much faster than Lynnfield and some of those newer CPU can fit into a 21.5" iMac ?