Insert with possible constraint violations?

["A B" <gentosaker(at)gmail(dot)com>]

Hello.
What is the proper way of inserting stuff into a table with two fields
(A and B) with a  constraint unique(A,B) and you suspect you will get
some unique constraint violations?
Should I just do INSERT etc...  and just ignore the answer?
Or do a select first and see if it returned any record?
Or is there a way of way to write a function that do the INSERT but
catches the error and do not write anything to the log about the
violation? In that case, what would such a function look like?
To me it seems that writing error messages  to log files  is not very
efficient.
So what would you suggest me doing?