how to improve my ways of handling errors than avoiding it?

[MSIE]

Let say i insert a record in my database

New Record

CustomerID 1111 --> Inserted Successfully

Next record

CustomerID 2222 --> Duplicated CustomerID --> Should show duplicated.

Right now i am using

IF EXISTS (SELECT ....)
BEGIN

INSERT
(...)
END

BEGIN
UPDATE(....)
END

Any ways to check for duplicated instead of updating it? Any examples?