I can reproduce the following behavior with emacs -Q.
When I use m-x man , it opens the man page in another window. When only one window is open, it even splits the frame. More annoyingly, when I follow a link in the manpage, it follows it in a different window, not in the window that displays the *man* buffer.
Is it possible to configure the man command in such a way that it always uses the currently selected window, without using any packages if possible. Basically, I want it to behave as if I would always use C-x 4 1 M-x man.