Everyone requires water. Yet, humans have evolved to need fresh water. However, we supposedly evolved from creatures, which came from the ocean. Could ancient civilizations have flourished from fresh ocean water. Yet, some war, or catastrophic event could have polluted the oceans with salt. Whole civilizations would have died out as the few that remained retreated to inland fresh water sources.

Fresh water is again becoming scarce. Will such a history repeat? Has it happened before?

it definetly was freshwater but probably before life started or when the very first lifeforms started
water runoff from rain slowly striped rocks of salt and other trace elements and eventually made there way to the ocean.
Ocean is still getting saltier until it reaches saturation