This is a little surprising since on CUPS systems we are now supposed to be using the PWG names for paper sizes which match known sizes, ignoring any information other than paper dimensions. I would have expected the "letter" entries to have had the same name because of that and the test for unique paper sizes to have removed the duplicate. That makes me think they are different in actual dimensions too.
Bug 1668794 Comment 5 Edit History
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This is a little surprising that you get both since on CUPS systems we are now supposed to be using the PWG names for paper sizes which match known sizes, ignoring any information other than paper dimensions. I would have expected the "letter" entries to have had the same name because of that and the test for unique paper sizes to have removed the duplicate. It does make sense they both show up as `US Letter` because of that, though. That makes me think they are different in actual dimensions too.