How to update one or more packages depended?

[Water NB <netbsd78%126.com@localhost>]

I need update a package A.
A is depended by B1 and B2.
Both B1 and B2 are depended by C.

if I run "make update" for A, will it build C twice? how to avoid it?
if I need update many packages A1, A2, ..., how can I avoid reduplicated
building.

thanks.