To clarify Smeghead’s remarks:
Crossing over happens a lot, on every chromosome. It is uneven. But it happens, on average, once per X length of DNA per meiosis. There’s a length of chromosome material called a centimorgan (cM). You will get one crossover event every hundred meioses in each cM of DNA, on average. The human genome is about 3000 cM long, so you get about 30 crossovers per meiosis.
It should also be noted that the female meiosis induces more crossovers than the male meiosis, and not just because of the extra X chromosome. I’m sure I learned the reason in genetics but I can’t remember it now 
LL
I can’t help much with the cousins, but I can say that “extremely hard to tell apart when together” isn’t much of a requirement when dealing with the general public. My brother is eight years my senior, with different eye and hair color. My father is 33 years my senior, again with different eye and hair color. I have lost count of the number of times various combinations of the three of us have been called twins or triplets, or that people who have known us for years will mistake one of us for another in the presence of all three.
I suspect that you could take any two random people (preferably of the same sex) off the street, coach them a bit, dress them the same, and have them walk around together and people would call them twins.
Er…sorry for the rant. This peculiar lack of perception has bothered me for years.
I’m sure I’m just confused, but don’t double first cousins have all four grandparents in common, while half-siblings only have two common grandparents. It seems to me that this would make double first cousins more closely related than half-siblings.
Czar Nicholas of Russia and King George’of England were cousins who looked extremely similar. The book “Nicholas and Alexandria” has a photo of the two of them, and you would think they were twins. All that royal interbreeding I guess.