Conditional Adding to a Table

Greetings,

I need to check a table to see if an item already exist in the table.
It if does not, then I want to add it.  Unfortunately, multiple processes
(Solaris system) can be running at the same time.  Sometimes, one process
checks to see if an item exist and it does not. At the same time another
process checks to see if the same item exist and it does not.  Then both,
add the item to the table.  This is, of course, not what I want.

I cannot seem to figure out how to do a conditional test to do something
like this.  If I could, then the whole operation could fit in one transaction
with locking.

Any ideas?

Thanks,
Glenn