数据库并发问题

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CREATE procedure [dbo].[spCopyData]

as
declare @corpid int

BEGIN
SET NOCOUNT ON
DECLARE @ordernumber int

BEGIN TRY

BEGIN TRANSACTION
UPDATE test SET
@ordernumber = ordernumber = ordernumber + 1
WHERE corpid = 15
INSERT INTO test1(id,ordernumber)
VALUES (newid(),@ordernumber)
COMMIT TRANSACTION

END TRY
BEGIN CATCH
ROLLBACK TRANSACTION

END CATCH
END

这个存储过程从test表取ordernumber +1，然后更新test表的ordernumber ，再插入到test1表里，但是当几个用户来访问的时候出现test1表的ordernumber 重复了，怎么解决这个问题？