I need a detailed answer to this question. Thank you!
The short answer would be: in the available sources (Snorri's Prose Edda and a couple of poems from the Poetic Edda) there is no mention of a second Ragnarok and the event is presented as unique, as implied also by the etymology "fate of the gods".
There are however hints of a cyclical nature of the Norse cosmos, which might suggest the possibility of a repetition of the history of the universe, including the final eschatological event. Such hints are:
- The survival of Lif and Lifthrasir, who will hide in a tree (probably Yggdrasill), and will repopulate the world of humans. This seems a lot like a second creation myth of mankind, a copy of the myth of the creation Askr and Embla from wood.
- The post-Ragnarok pantheon, which can be seen as a double of the pre-Ragnarok one, with Thor's sons carrying Mjolnir, Odin's sons Vithar and Vali taking his place, and Baldr and Hodr coming back from Hel.
But the passage that has been used the most to argue for a cyclical nature of the Norse universe is the following, taken from the Voluspa (translation by Carolyne Larrington):
There comes the shadow-dark dragon flying,
the gleaming serpent, up from Dark-of-moon Hills;
Nidhogg flies over the plain, in his pinions
he carries corpses; now she will sink down.
This passage is the last stanza of the Voluspa, and it comes right after the description of Gimle, the glorious hall where all surviving gods will dwell after Ragnarok. The interpretation of this stanza has been much debated. If we retain the reading of the main available manuscripts, the sentence "now she will sink down" should refer to the seeress (the narrator of the Voluspa), who is about to disappear after having completed her prophecy. According to this intepretation, the reference to Nidhogg has been interpreted as an indication that even in the post-Ragnarok world evil will be present, and possibly another clash between the forces of good and evil will have to occur.
But this is just one possible interpretation of the stanza. Nidhogg could just be a narrative device that brings us back to the pre-Ragnarok world of the seeress, where evil still exists. This is what Larrington says about the stanza:
In the final verse the sinister dragon Nidhogg is seen as the seeress sinks out of her trance. Does evil still exist in the new world, or have we returned to the present where the dragon is a portent of Ragnarok?
Moreover, another school of thought renders the final sentence as "now he will sink" (see Lee M. Hollander's translation of the Voluspa). In this case it would not refer to the seeress, but to the serpent itself, who disappears in the new order of things.
I would like to add one more argument against the cyclicity of the Norse universe. The post-Ragnarok era is presented as an out-of-time golden age, in which "crops will grow unsown" (from Anthony Faulkes's translation of the Prose Edda), "all evil will be healed" (Larrington's translation of the Voluspa), and the gods will "live forever in ease and bliss" (Hollander's translation of the Voluspa). Such a description is in deep contrast with the pre-Ragnarok world, where both gods and men are in constant struggle with the forces of evil.
Some people here are speaking about Raganarok being cyclical. That's definitely one interpretation, but another interpretation that I was impressed by was that it already happened, that Ragnarok is a story of how our world came to be. Think of it, the man and woman who repopulate the world after taking shelter under Yggdrasil sound a lot like Adam and Eve.