Amer asks two interesting questions:
1. I am not sure about the following sentences:
i) They have once again won
ii) They have won once again
Which one is correct?
Actually, both are correct. But here is an odd fact about English. We tend to pay the most attention to the beginning and end of a sentence, and the beginning and end of a paragraph. I call these the "hot spots" of sentences and paragraphs. We read the words in between, but they don't have as much impact.
This is why a "bad news" business letter usually puts the bad news in the middle of a paragraph, with some neutral information at the beginning and end. The bad news doesn't hurt as much.
So in the sentences you ask about, it depends on what you want to emphasize: If it's "won," then leave it at the end. If you want to emphasize that they have won "once again," put that at the end.
2. Can you end a sentence with a preposition?
Yes. We end sentences with prepositions all the time, in both speech and writing.
Once an editor corrected something that Winston Churchill had written, saying that Churchill should not end a sentence with a preposition. Churchill wrote back: "This is the kind of hairsplitting up with which I will not put." He didn't end the sentence with a preposition, but it sounded silly.