system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] Quick question about time travel

I'm a grad student researcher who has been helping to get current events added to the history textbook revisions coming out next year, and my team is covering the Battle of Earth and the events of the Blip. 

From what I've been able to verify, there was some kind of time travel(!) involved in which members of the Avengers had to move backwards in time in order to collect essentially magical stones in order to put them all together in a gauntlet to undo the Decimation. A leaked document seemed to indicate that after the end of the battle, Steve Rogers moved backwards in order to return the stones, lest he create some kind of paradox whereby the stones couldn't have been used to begin with. Dr. Banner described it as "the impossibility of changing the future by changing the past" at a lecture at NYU earlier this year. 

But Thanos, his Black Order, and a fairly large ground invasion force were apparently killed twice. The Black Order and much of his invasion force were killed in the Battle of Wakanda, and Thanos was beheaded by Thor. They were also all (presuming the same ground invasion force) decimated in the Battle of Earth. They died twice, and the versions of them at the Battle of Earth were not returned to the past so they could carry out the Battle of Wakanda and the Decimation. 

I'm only a historian-in-training, so maybe I'm missing something, but I can't seem to square this circle, I have a publishing deadline coming up, and I'm already 5 years behind my life plan to become a history professor. Any help would be greatly appreciated.
gpt
When an event of this nature occurs, the time-path splits, temporarily creating a new reality. Using secrets from the captured Nebula, a Thanos from the past created one of these splits and came to the future of earth to grab the stones, a similar action to what the avengers did to re-collect them but in reverse. In a manner of speaking Thanos didn't die twice, two Thanos' died once, the second one's time branch was then trimmed by Steve Rogers so events there were prevented from having changed, but our main timeline still bears the scars of the others influence in the tragic effects of The Snap and the battlefield where the Avenger's northern base used to be.