Cannot insert duplicate key row in object '%.*ls' with unique index '%.*ls'.

Error Message:
Msg 2601, Level 14, State 1, Line 3
Cannot insert duplicate key row in object '%.*ls' with unique index '%.*ls'.

Severity level:
14.

Description:
This error message appears when you try to insert an already existing value into a unique key column.

Consequences:
The T-SQL statement can be parsed, but causes the error at runtime.

Resolution:
Error of the Severity Level 14 are generated by the user and are corrigible by the user. The statement cannot be executed this way. Duplicate key values are not allowed in a unique key column. The data cannot be inserted.

Versions:
All versions of SQL Server.

Example(s):
USE tempdb;
IF OBJECT_ID('tempdb..#t') > 0
 DROP TABLE #t
CREATE TABLE #t
(
 c1 INT
)
GO
CREATE UNIQUE INDEX UI_t_c1 ON #t(c1)
GO

INSERT INTO #t SELECT 1
INSERT INTO #t SELECT 1

Remarks:
In the above example we try to insert twice a value of 1 into the column c1 of the table #t. Because there exists a unique index for this column, the error is raised.