Duda was on optional assignment with AAA Buffalo at the time of the call-up, so he was already on the 40-man roster. He has spent (approximately) 58 days on optional assignment in 2011, so the option year will officially count as his first. This call-up, as well as any subsequent 2011 options and call-ups, will have no effect on the number of options he has remaining.
Evans is out of options, so to clear the spot for Duda on the 25-man active roster, he was designated for assignment (DFA'd). In addition to removing him from the active roster, the DFA also removes him from the 40-man roster. The Mets now have 10 days to trade him, release him, or send him outright to the minors. If they wish to outright him, he will first need to clear waivers. Additionally, since a player can only be outrighted once in his career without his consent, Evans will have the right to refuse the assignment. If he exercises that right, he will immediately become a free agent and will forfeit the remainder of his contract. If he accepts the assignment, he will be eligible for free agency at the end of the season (unless he is back on the 40-man roster at that time).
Regardless of what happens with Evans, this move reduces the Mets' 40-man roster count from 40 to 39. The open spot will not be filled until the Mets choose to either reinstate a player from the 60-day DL or call up a player who is not currently on the 40-man roster.