select: epoll.register(): Handle case of fd have been already registered.

Is this correct enough? Certainly, Python's epoll should not be fire-once?

select/select.py

@@ -37,6 +37,8 @@ class Epoll:
             retval = fd
         s = struct.pack(self.epoll_event, eventmask, retval)
         r = epoll_ctl(self.epfd, EPOLL_CTL_ADD, fd, s)
+        if r == -1 and os.errno.get() == 17:
+            r = epoll_ctl(self.epfd, EPOLL_CTL_MOD, fd, s)
         os.check_error(r)
 
     def poll(self, timeout=-1):