duplicate key was found for the object name 'dbo.syscolpars'

Question

ALTER DATABASE testDatabase SET SINGLE_USER WITH ROLLBACK IMMEDIATE; 

GO 

ALTER DATABASE testDatabase COLLATE SQL_Latin1_General_CP1253_CI_AI; 

GO 

ALTER DATABASE testDatabase SET MULTI_USER; 

GO 

Error :

The CREATE UNIQUE INDEX statement terminated because a duplicate key was found for the object name 'dbo.syscolpars' and the index name 'nc'. The duplicate key value is (397960494, Categorie, 0).
Msg 5072, Level 16, State 1, Line 5

I'm trying to alter the database collation, and i'm getting this error, i don't understand why, tried to select the dbo.syscolpars, but i'm unable to/don't know how, please enlighten me !!