This may be considered an exploit. I think there are probably some new players now versus the last time I posted this, but I've tested it and it still works:
At various point in tech trees, you have to choose between several enhancements to an existing tech, such as choosing to make a particular type of ship component cheaper maintenancewise, a bonus to what it does, or miniaturized (giving it lower mass so you can put more of it on a ship). On the tech tree, you can recognize these techs by the rounded ends instead of rectangular.
Even though you're only allowed to choose one, and once you choose it (or acquire one through a tech trade) you cannot research more, if you are strategic and careful, you can get more than one with careful tech trading.
Generally, let other Civs research the enhancement first; do NOT trade to get the choices they make right away. Instead, go through them all, THEN research the tech choice none of the AI have made. You can then, after researching your enhancement, trade with the various AIs to get the alternate enhancements. I am generally able to get them all this way.
It can help to trade or even gift them the base tech (before the enhancement) if they don't have it already.