There is an assumption that the author of the contradiction makes which I can not verify. That assumption is that 3:42-45 and 19:17-18 are the same occasion. Is it not reasonable that Mary (may God be pleased with her) was informed of what was to happen more than once? Keep in mind assumptions which do not contradict the Qur'an and allow an uncontradictory solution, like the one I am entertaining with respect to the number of times Mary was informed angelic beings, may not be the actual truth (and I am not saying that Mary was told this information once, twice, or many times), but nevertheless those assumptions which illustrate that a contradiction has not been proven are just as valid as assumptions which lead to contradictions. Therefore, the contradiction has not been proven at all, and with that I leave it with the following: God knows best how many times Mary was told this information and who told her.
So according to the criteria of proving a contradiction, the contradiction author's argument fails. If the reader is wondering what the criteria of proof of contradiction is, see my response to this web site's page on "Heavens or Earth, Which was created first?". In that response I detail how to determine whether or not a proposed contradiction has proven its point.