When the Almighty Allah is One, unique and peerless, and knows about His oneness, then why in the Holy Quran he uses the plural pronoun (We) for Himself?
In the Arabic, Farsi and Urdu language the plural nouns are normally used to denote respect.
When Allah talks about Himself, His names, His attributes(siffat), His essence; He talks about these things with a language of respect, using the plurals.
This is just how language is. And when this is translated into the English language the term "We" or "Us" is used.