As far as stealing techs goes, there have been a fair number of threads on the apparently odd behavior involved with it. My theory is that you always "steal" a tech...but sometimes it's a tech you already have. In essence, when you invade a planet, one of the defending AI's techs is chosen for you to steal, at pseudo-random, and if you already have it, nothing happens. I don't know that this is valid, but it fits with the observed behavior of tech stealing.
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I used to think the same thing, but I don't think the statistics bear this out. It seems more like a 50% of a steal, and then it is randomly chosen from the techs you don't already have. I say that because if it's the way you're describing, the odds of stealing a tech would fall way way off later in the game.
Usually late in the game you'll get the one tech you could get for example.
Me personally, I'd love for them to add a miny game for the tech steal, and perhap not allow you to research further from a stolen tech until you get it's pre-reqs.