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This happens a lot. In French they use "vous" (pl) instead of "tu" (sing) when being polite or respectful (see tu versus vous). Russian uses "ty"/"vy" in the same way. Basque uses the plural "zu" for the formal singular and they put the plural suffix "-ek" onto that to make it formal plural "zuek". In English we used to use "thou" for singular and "you" or "ye" for plural. Then we started using "you" for polite singular, and then we just assumed everyone might as well be polite and threw out "thou". Now we use "you" for both singular and plural. Kings and queens refer to theirselves as "we" (royal "we").

My theory for why this happens is that if someone is singular, they're just one person and they can have no impact on the world, so it's a little bit insulting, but if someone is plural, they have friends, there is more than one person on their side, so they must have some power and influence. That's just my theory, though, so take it cum grano salis.

If anyone has an example of this happening in a non-Indo-European language (besides Basque) or with third person pronouns, please send me a msg.

wow just realized most of this is already noded at tu