Msg 1915, Level 16, State 1, Line 1
Cannot alter a non-unique index with ignore_dup_key index option. Index ‘%.*ls’ is non-unique.
This error message appears when you try to use the IGNORE_DUP_KEY option for a non-unique index.
The T-SQL statement can be parsed, but causes the error at runtime.
Error of the Severity Level 16 are generated by the user and can be fixed by the SQL Server user. The statement cannot be executed this way. The IGNORE_DUP_KEY option cannot be used for a non-unique index.
All versions of SQL Server
CREATE TABLE #t
CREATE INDEX UI_t_c1 ON #t(c1)
ALTER INDEX UI_t_c1 ON #t
SET (IGNORE_DUP_KEY = ON);
DROP TABLE #t;
In the above example we try to use the IGNORE_DUP_KEY option for the non-unique index UI_t_c1. This raises the error.