This goes like this: 1. You need a city belonging to an extinct civ that is flipping into a free city, that city needs to be under the loyalty pressure of a 3rd civilization, enough to flip it. 2. You capture the free city with your military and choose to "liberate" it, granting you 300 favors. 3. The city then returns to loyalty pressure from the 3rd civ, which will flip it back into a free city. 4. Repeat step 2 again, in order to farm for 300 favors every several turns. I call it an exploit. You should never be granted favors for liberating the same city for the second time within the same era as an example for a solution. This is an old exploit, I have seen it multiple times since the beginning of the game, but I just found it now to be annoying enough to report it for fixing.