When running M-x man RET man RET, then C-x o to select the newly created
window and then running M-x man RET sed RET, what you get is: two "man"
windows side by side. It would make sense to re-use the existing one.

How to best do that I do not know. If modifying the code is necessary,
here's an attempt at adding an option in Man-notify-method. (I'm not
attached to this implementation, and even less attached to the silly
name `manly' that I chose, btw.)