As near as can be told the ruins do not generate a random tech, thier tech is set at the time of galaxy creation. If you already have what they would give you, then you get nothing.
I haven't experimented with this myself, but I mentioned the problem on the SE4 mailing list and a few people who had messed with it report this to me.
I do know that I have never had more than one special tech in any given game, and that I never yet have gotten anything at all out of the fourth and later ruins I explore.
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