I was reading the book Spice and Wolf and I saw a sentence like this:
…尻尾を左右に振っていたのだった。
I have never seen this kind of "double past tense" before. All the past tense sentences I have seen that uses the のだ grammar always end in のだ, not のだった. 振っている is already turned into past tense 振っていた, so why use だった instead of だ?
In other words, what is the difference between the above and the below?
…尻尾を左右に振っていたのだ。
The two versions mean the same thing, and I don't realize any significant difference between them. Maybe, the first one emphasizes that the time is the past. I mean, the first sentence emphasizes the past tense. However, as you mentioned, the second one is perfectly fine in this context.
By the way, I've encountered what you called the "double past tense" so many times in Japanese so far. I don't think it's ungrammatical or weird. You will see it quite often, I believe.
Now my concern is the style of the novel. If the novel is written in the "narrative mode," I mean the total sentences are basically written in the present tense, it would be natural to choose the present tense at the end of this sentence as well. However, if the total sentences are basically written in the past tense, you may think it's more natural to choose the past tense in the end of the sentence.
