When you forget to press the Symetry button...

[David Saber]

I’m modeling shapes symetrically, so I can’t use “symtrize polygons”.
Sometimes I forget to press the “sym” button , so I have to redo the same work 
on the other half.
Is there a way to tell the points on the “forgotten” half to adapt to what was 
done on the modeled half?
Thanks
David