Do "Bonus extending" building (Factory, Zoo, ...) stack?

Question

In the mid-game of Civilization 6 you receive access to buildings like the factory or the zoo which have (in addition do some other bonuses) the attribute:

Bonus is extended to all cities within 6 tiles

Let's say city A has an industrial district which is at or closer than 6 tiles to city B and - similar - city B has an in industrial district close enough for city A. Will both Cities get the effect of one industrial zone*, or will both cities get the effect twice?

*Related question: I'm assuming "Bonus is extended" refers the all bonuses of the district. (The district itself plus all buildings inside, like the workshop) Is this correct?

Answer 1

EDIT: As of the Winter 2016 update, it seems it no longer works the same way. From the patchnotes:

Cities can no longer receive yields from more than one regional building per type; they take the highest (ex. production from multiple Factories)

Cities can no longer receive amenities from more than one regional building per type; they take the highest (ex. amenities from multiple Stadiums)

Answer BEFORE Winter 2016 update:

First off, you can actually directly see the various bonuses by hovering over your city's production. (same for food, faith, science, etc.)

I found out that

A) The bonus does stack, a factory gives +3 production, a city with a factory of its own within range of another factory has +6 instead of +3, thus receiving the bonus.

EDIT : This bonus CAN stack multiple times:

As you can see here, my Capital is within range of 3 different fully upgraded Industrial districts, so it receives +9 from factory (+3 from each) and +12 from power plants (+4 from each).

B) Only the factory bonus was added to cities within 6 tiles, NOT the whole zone bonus. So say a city with no industrial district is in range of an industrial zone with a factory and a workshop, the other city will only receive the +3 bonus from the factory. Again, as you can see in the screenshot, my capital does not have an industrial district of its own, so it only receives bonuses from Factories and Power plants, not from the workshops.

This makes sense as the entertainment complex, for example, has multiple buildings that extend their bonuses. So each building is worth building, as each will only give off its own bonus.

Answer 2

As of 12/20/2016 it no longer stacks, the Winter Update changed/fixed this.

Answer 3

So far what i've experienced, is that the different bonuses stack, but im not sure whether its the building with the tooltip that is extended to 6 tiles or if its the whole district.

also im not sure if its the district that is the center of the extension or if its the city that owns the district? perhaps you can elaborate me with an answer to this?

PS: around mid game i tend to build industrial districts in all cities, and entertainment districts here and there when needed, to get that late game amenity boost.