Sometimes it so happens that I forget whether I have wiped over my head or not, and I am not sure one way or the other. Should I do wudoo’ all over again?

Praise be to Allaah. 

If this uncertainty come after you have finished
doing wudoo’, then do not pay any attention to it. If it happens before
you finish, for example, being unsure whether you have wiped over your
head and you are washing your feet, then you can wipe over your head
and wash your feet. There is no hardship involved in doing that. This
is if you are not  suffering from a lot of insinuating doubts.
If you are faced with a lot of insinuating doubts, then you should not
pay any attention to that, and you should proceed from the point at
which you are now. So if you are washing your feet then you may assume
that you have wiped over your head. The same applies to all other stages
of wudoo’.