UPDATE IF EXISTS, otherwise INSERT

yesterday it came to our attention that our unique id wasn't remaining unique upon insert, and certain orders were being rejected.....

our unique id is based on eventtime,eventtype, orderid
the records themselves were distinctly different, but SQL's little timestamp accuracy problem was causing the eventtimes to round.  thus, becoming consistent w/eventtimes already in the table, and causing duplicate constraint violations.

so.  i changed the proc to do an IF/EXISTS.   if it exists, it should be updated.  otherwise, it should be inserted.

today i'm getting the same violations.  i don't know if it is me, or the app, so i was hoping to confirm whether my logic is flawed.  


CREATE PROCEDURE procname
(
   @EventType char(2),
   @EventTime datetime,
   @gwid int,
   @MessageID varchar(32),
   @OrderID varchar(32),
   @ExchangeID varchar(32),
   @EndPoint varchar(8),
   @UserId varchar(16),
   @AcctId varchar(16),
   @Client varchar(32),
   @Symbol varchar(16),
   @Price decimal(18,8),
   @Side char(1),
   @Quantity bigint,
   @OrderMarking char(2),
   @FirmPosition bigint,
   @TimeInForce varchar(5),
   @ExecInstruction varchar(10)=NULL
)
AS
SET NOCOUNT ON
DECLARE @error INT

IF EXISTS(SELECT 1 FROM database.dbo.table1 WHERE GWID = @gwid)
BEGIN
      IF EXISTS(SELECT 1 FROM database.dbo.table2 WHERE MessageID = @messageID AND OrderID = @orderID AND EventType = @eventType)
      BEGIN
            UPDATE database.dbo.table2
            SET EventTime = @eventtime,
            Quantity = @quantity
      END
      ELSE
      BEGIN
            INSERT database.dbo.table2 (
            ....all the fields....
            )
            VALUES (
               .....all the @fields.....
            )
            END
            IF @@ERROR = 0 OR @@ROWCOUNT <> 0  -- thus, successfully inserted
                 RETURN 0
END
            -- on failure run this
            INSERT INTO database.dbo.failuretable (.......all the fields...........)
                  VALUES (.........all the @fields..............)

            RAISERROR (' Events failure upon insert.  MessageID is %s.',16,1,@MessageID) WITH LOG

SET NOCOUNT OFF
GO