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Most repeated errors are shown as many times as they occur, but that's not the case with invalid attribute names. This is true in 0.7.4 and 0.8.0b1.
Created attachment 465 [details] Test case
This is indeed a known (and to me, puzzling, some may know more context) behavior of opensp, the SGML parser used by the validator. I am not aware of any option to force opensp to output all the errors, and not consider some duplicate uninteresting.