http://www.freepatentsonline.com/5608735.html two same-type-ads shouldn't be in the same page as far as I know. moreover, it's position in Alexa is high, I don't know how can they do such a mistake.
Two same-type-ads can be in the same page as far as I know. Where did you saw that this is not allowed?
I agree, 2 same types can be used on same page. You could have all three the same type if you wanted. Just not more than three max of one format. when I say format I mean either link, or content types.
2 types of identical ads can coexist on the same page. The only restriction is the number of ads you can place, ie 3 regardless of the size.