SQL Server UNIQUE失效

CREATE TABLE dbo.Code_Index
(
code SMALLINT IDENTITY(1,1) CONSTRAINT PK_Code_Index_code PRIMARY KEY CLUSTERED (code ASC) WITH(PAD_INDEX=ON,FILLFACTOR=80,ALLOW_ROW_LOCKS=ON,ALLOW_PAGE_LOCKS=ON),

item VARCHAR(20) NOT NULL CONSTRAINT UQ_Code_Index_code UNIQUE NONCLUSTERED (code ASC) WITH(PAD_INDEX=ON,FILLFACTOR=80,ALLOW_ROW_LOCKS=ON,ALLOW_PAGE_LOCKS=ON)
)

以上命令建立的表格,依旧能够输入重复数据为什么?
注:输入重复数据是,提示提交数据错误,此数据为只读,表中依旧有两条记录.

CREATE TABLE dbo.Code_Index ( code SMALLINT IDENTITY(1,1) CONSTRAINT PK_Code_Index_code PRIMARY KEY CLUSTERED (code ASC) WITH(PAD_INDEX=ON,FILLFACTOR=80,ALLOW_ROW_LOCKS=ON,ALLOW_PAGE_LOCKS=ON),   item VARCHAR(20) NOT NULL CONSTRAINT UQ_Code_Index_code UNIQUE NONCLUSTERED (item ASC) WITH(PAD_INDEX=ON,FILLFACTOR=80,ALLOW_ROW_LOCKS=ON,ALLOW_PAGE_LOCKS=ON) )   INSERT INTO Code_Index(item) VALUES ('a')   INSERT INTO Code_Index(item) VALUES ('a') SELECT * FROM Code_Index

CREATE TABLE dbo.Code_Index ( code SMALLINT IDENTITY(1,1) CONSTRAINT PK_Code_Index_code PRIMARY KEY CLUSTERED (code ASC)  WITH(PAD_INDEX=ON,FILLFACTOR=80,ALLOW_ROW_LOCKS=ON,ALLOW_PAGE_LOCKS=ON),   item VARCHAR(20) NOT NULL CONSTRAINT UQ_Code_Index_code UNIQUE NONCLUSTERED (item ASC)  WITH(PAD_INDEX=ON,FILLFACTOR=80,ALLOW_ROW_LOCKS=ON,ALLOW_PAGE_LOCKS=ON) )