how to change collation for databse

Hi all,

when  I try to alter collation type is shows up below 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 (327672215, @PID, 1).

ALTER DATABASE testdb
COLLATE Latin1_General_CI_AS

default was Latin1_General_BIN .

Can anyone help me in this pls.

Ta,