As reported in #5199 and fixed in the GUI in D1558, when doubleclicking on the StartGame button of the gamesetup as a host, launchGame of gamesetup.js will be executed twice.
This will cause the NetServer to start the game, then compute a new RNG seed and broadcast that. So the mapgen is done with the first seed but the simulation uses the second seed.
Then rejoiners will compute the mapgen with the first seed.
This patch fixes this issue more at the root of the problem and doesn't rely on the JS GUI being sane.
Similar to D1513 and D1556, we notice that splitting the PREGAME state into GAMESETUP and LOADING might have prevented this issue.