It would indeed be extraordinary if City could repeat last season's achievements. the first leg against Copenhagen was in their favor. and the second leg they will still be favorites to win.
If you meant winning the treble again, then yes it would be unbelievable. City won their first CL title last season and made it even more special by winning the treble, but to do that all over again this season, something that has never been done, then that would be an unprecedented feat. Pep's men already have one foot in the CL QF, and they are still alive in both the EPL and the FA cup, so yes, anything can happen and if it does, it would cement Pep's and his players' legacies in football's history.
It is very difficult but it is still possible to happen, seeing Manchester City's performance which has been increasing or consistent since the start, they certainly have big ambitions to make new history.
Against Copenhagen in the second leg will not be a problem and indeed Manchester City is almost certain to advance to the quarter-finals but for the next opponent I think it will not be easy, what remains now is the top European team which has very balanced quality so that's enough. it is difficult for us to make predictions early on.
What must also be remembered is that only Real Madrid is able to win the Champions League in a row, the Manchester City team which has the strength of course wants to do the same thing.