空表INSERT [sql-server]抛出UNIQUE约束异常

UNIQUE constraint exception thrown on empty table INSERT [sql-server]

我的INSERT语句在尝试将新记录添加到空表(属性)(尚未记录)时失败。
我对系统引发的错误感到惊讶：

Violation of UNIQUE KEY constraint 'CK_Attribute_Name_IDproject'. Cannot insert duplicate key in object 'dbo.Attribute'. The duplicate key value is (dummy, 55).

该表的创建脚本如下所示

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CREATE TABLE [dbo].[Attribute](
[ID] [int] IDENTITY(1,1) NOT NULL,
[IDproject] [int] NOT NULL,
[IDtype] [int] NOT NULL,
[IDgroup] [int] NOT NULL,
[name] [varchar](50) NOT NULL,
[color] [int] NULL,
[protected] [tinyint] NULL,
[datemodified] [datetime] NOT NULL,
PRIMARY KEY CLUSTERED
(
[ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
CONSTRAINT [CK_Attribute_Name_IDproject] UNIQUE NONCLUSTERED
(
[name] ASC,
[IDproject] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

我滑过外键引用和默认值，这在这种情况下似乎并不重要。
UNIQUE约束适用于[name]和[IDproject]。

运行以下语句时

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SELECT *
FROM [dbo].[Attribute]
GO
SELECT *
FROM [dbo].[Project]
GO

我得到了结果

(0 row(s) affected)
(2 row(s) affected)

第一个结果表明属性表为空
第二，有2个项目

然后在表Attribute中运行以下INSERT，它失败并出现上述UNIQUE CONSTRAINT错误

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INSERT INTO [dbo].[Attribute] ([IDproject], [name], [IDtype], [IDgroup], [color], [protected], [datemodified])
SELECT DISTINCT
p.[ID],'dummy',t.[ID],g.[ID],-1,0,getdate()
FROM [dbo].[Project] p
INNER JOIN [dbo].[Group] g ON g.[name]='none' AND g.[IDproject] = p.[ID]
INNER JOIN [dbo].[AttributeType] t ON t.[format]='text' AND g.[IDproject] = p.[ID]
WHERE p.[name]='TESTPROJ'
GO

我如何在空表上得到这样的错误？