system
Given the following post, respond as if you're commenting on Reddit:
human
[40K, 30K] The Imperium and Galaxy of the 30th Millennium are flung forward into the 40th Millennium.

The entire 30K galaxy and all factions and personalities are flung into 40K time and space, but just adjacent to the 40K galaxy.  There are literally two milky way galaxies next to one another, stacked like pancakes.  Two of each planet, etc.  (Ignore the large scale physics issues, unless it leads to a particularly interesting result.)

Setting:


1)  That the 30K Imperium is at the height of it's power, just before the triumph at Ullanor.

2)  Horus hasn't felt abandoned and the seeds of discontent have not been sown.  The 30K Imperium is still totally loyal.

3)  Magnus has not been humiliated at Nikea.

4)  Lorgar has not been humiliated at Monarchia.

5)  Chaos has not yet had any overt imfluence on the Primarchs or Fabricators.


So what happens?


1)  What happens when the 30K Primarchs and the Emperor are confronted with the knowledge of the events of the Horus Heresy?

2)  Does the Emperor accept the 40K Imperium as redeemable or is war inevitable?

3)  Which factions come into conflict with their future selves?

4)  What interesting paradoxes develop?
gpt
The sudden realization that the Emperor was hiding the nature of the Warp from just about everyone and their mother would be detrimental. The Primarchs would feel betrayed (even those that were loyalists in the 'primary' timeline) and would likely start doubting their Father. The Ultramarines went so far as to propose a Second Empire when they thought that Terra had been lost, and Guilliman may propose such again.

You also have Lorgar, who was corrupted by Chaos well, WELL before the Heresy started, maybe even before the Emperor found him. Monarchia was just the trigger, Chaos was there the whole time, lurking in the back of his mind.

The Church of the Savior Emperor would see this as a miracle, and rally the majority of the 40k Imperium behind the newly re-emerged Emperor. 

The Eldar would be rather ecstatic, because A; Eldrad would be back. B; Iyanden would ever have been ravaged by the Hive Fleets, and C; their population suddenly more than doubled.

The Orks would be... Orky

Chaos would, likely, be largely unaffected.

The Necrons would be... Torn. On the one hand, they have thousands of new tomb worlds to activate. on the other... Dynastic conflicts between duplicates. Imagine two Imotek's duking it out.


This type of duplication is actually rather common with the Warp, though never to this scale.