This is not useful, yet, but if it is understood or people test a lot, it might become useful.

This is what I did, in detail - alternatively, watch the video and notice how I start Taxi Driver during Taxi Driver

Now I get a destination immediately after starting the mission. It is always the cop station in Washington Beach but if you are in the same zone (probably that is what is about) then you get a random destination. The fare counter is only increased by one when I reach it, though. The police station is near the Malibu club and at the edge of the zone it is in so that the club is outside it but it is still too slow - iirc I times 35s with a Cabbie, that would take 35¤100 = 3500 ~ 60min. Feel free to time again with optimal execution and the Zebra Cab.

When I save and load after "double-starting" taxi and then vigilante start Taxi Driver, the instance with the police station destination fails and iirc the other one doesn't increment the fare counter.

Duping taxi before is not necessary. It is enough to start and cancel it, then do vigilante taxi start.

In fact it's the same technique that is used in instapasses. Taxi mission is just skipped to the point when a destination is generated and it happens to be an offset corresponding to Washington Beach. The only thing that can fail is the fact that due to a skip game originally doesn't know which car Tommy uses. This value is initialized by regular starting taxi driver and cancelling it. The only condition is that the car that was used for original taxi start is still alive, at least for a few frames before the other copy of taxi driver finds player's vehicle.

Also it counts as one fare since the other copy of taxi driver works as intended and waits for chosen ped to enter cab.

since we're talking about weird taxi dupes and you seem to be able to answer anything, can you explain this http://www.twitch.tv/gaeldemarseille/c/6087877

taxi dupe; 1st fare = 2, 2nd fare =1 then 3rd fare = 2 again

Also it counts as one fare since the other copy of taxi driver works as intended and waits for chosen ped to enter cab.

so could you dupe this if you were to pick up a regular fare then take them to the cop station?

Nope, the destination is over-ridden by the fare's destination.