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.