So, according to the lore, both the Dovokiin and Alduin came back at the same time. The only way to kill a dragon permanently is for the Dovokiin to absorb the dragon's soul, right?

So, if the dragons were defeated long ago by another Dovokiin, how could they come back if their souls were absorbed? Wasn't Alduin defeated by a Dovokiin and a bunch of conspiritors trying to stop the enslavement of man/Elf kind?

Not all of the dragons were killed by the dragon born back then. Also maybe Alduin is giving his dragon servants new souls. My theory is that he resurrects dragons by stealing souls from svongarde and other Aurbis realms, and gives them to his dragon army.

Always nice to have access to any soul you want.

It was my understanding that the ancient 'Tongues' that defeated Alduin the first time weren't Dragonborn, hence why they weren't able to actually, truly defeat him, and had to use the Elder Scroll. It was a massive rebellion of the ancient Nords, who were taught the Thu'um by Paarthurnax (under orders from Kyne, or something like that). They were basically the predecessors of the Greybeards. Note that in the vision you get from the Elder Scroll, the dragon you see the woman killing doesn't have its soul absorbed.

This is why Alduin can resurrect all those other dragons. They never had their souls absorbed, they were just killed by regular mortals (and a lot of them by the ancient Blades). Presumably any that were actually killed by a previous Dragonborn aren't brought back, or at least there's no evidence that they were.

This backstory makes for a great possible DLC doesn't it?

"Thought Alduin was it?... You thought it was finished?... It has only just begun!"