I read through this multiple times trying to sort out for myself all the apparent confusion. At this point in time, this is what I think (maybe??) is a good explanation (if I understanding correctly)...
You can have 1-3 recurring quest slots depending on Age & circumstance
In every such circumstance, no specific recurring quest can be displayed at the same time in each recurring quest slot. For example, if coin collection is showing on one slot, it's impossible for it to show on the other slots. This is because the pool of recurring quests specific for your Age is a single grouping for the entirety of all quest slots to draw from, and there is only one copy of each quest in the general quest pool. Thus, if one slot is using it, then it's unavailable to be used by the other slots - when you scroll through the recurring quests for slots 2 or 3, specific quest(s) already being used won't be displayed as an option.
Except in OF!!
In OF, that Age's general quest pool has multiple copies of "collect 6, 24 hour productions". In fact, so many copies that all 2 (eventually 3) recurring quest slots can each use that specific quest simultaneously if you so choose.
When you describe having the 2 recurring quest slots, you note seeing 3, 24hour production quests when you scroll through each slot - which gives you the 6 total you describe (3 quests x 2 quest slots). What you saw is actually not 6 copies of that specific quest. There are only 3 copies in total in the general OF quest pool. But since you didn't have quest slot #1 set at 24hour production, those same 3 specific quests will also show up as being available for quest slot #2 (if you had slot #1 set at 24hour production, you would've only seen 2 copies of it when you scrolled through quests in slot #2).
Now you've progressed to having 3 recurring quest slots. And look... your general quest pool has 3 individual copies of that specific "collect 6, 24 hour productions" quest. Each of the 3 quest slots can display one of the 3 copies - thus, all can show the 24hour quest at the same time. Three times the recurring quest prizes for doing only one quest!
The other OF-only quest exception (that no other Age has) is that the "collect 6, 24 hour productions" does not specify which particular Age/building it must come from - every other Age makes you complete production quests from buildings of the same Age. Since OF doesn't have an age-specific requirement, some folks use a sea of blacksmiths (the smallest/cheapest/lowest population production building in the game).
Personally, my guess is Inno intended to have 3 copies of that production quest. But not making it for an OF production building only was likely a mistake.