Homonyms :) (like in english rose the color, rose the flower, to be right/to turn right ...) http://grammar.yourdictionary.com/for-students-and-parents/words-with-multiple-meanings.html Sei in italian is the number 6, but means also "you are" depending on the context. In sms language, young people write the number 6 to shorten the verb 'are', using the fact that those words are homonyms ;)
because unfortunately for some time (half a year or so?) there is a bug that Duo doesn'rt recognise anymore every mistake or typo.If it's occuring again, report it. There is a button "report"and there you can choose "should not have accepted my answer". It might be better to file a bug report though (look under the help button) . In any case, if you are not 100% sure you did it right, I recommend you check your answer with the sentence discussion or your notes.