Check before INSERT INTO

My table structure

CREATE TABLE my_shevi
(
id text,
n_gen serial NOT NULL,
n_sheet serial NOT NULL,
tot_sheet serial NOT NULL,
CONSTRAINT my_shevi_pkey PRIMARY KEY (n_gen, n_sheet, tot_sheet)
)
WITH OIDS;
ALTER TABLE my_shevi OWNER TO postgres;

The user updates the DB via ASP. When 2 users click on the submit button at the same time, only 1 record is inserted. (ERROR: duplicate key violates unique constraint "my_shevi_pkey")

For example they both send a string like below.
strSQL = INSERT INTO my_shevi VALUES ('a', 4, 1, 1);

I thought of adding a test before executing the insert into.

Set SQLN_GEN = oConn.Execute("SELECT upper(N_GEN), upper(N_SHEET), upper(TOT_N_SHEET) FROM " & TableName & " WHERE N_GEN='" & n_gen & "' AND N_SHEET='" & n_sheet & "' AND TOT_N_SHEET='" & tot_n_sheet & "'")

if (not SQLN_GEN.eof) then
***** set n_gen + 1
else
***** leave n_gen the way it is
end if

conn.Execute strSQL

But how can i ask it to change the n_gen value??? (The part with the *****)

Thanks

Shavonne Wijesinghe