La parte importante è IGNORE_DUP_KEY = ON

CREATE UNIQUE NONCLUSTERED INDEX [uniqueid] ON [dbo].[tabella] 
(
    [campo1] ASC,
    [campo2] ASC
)
WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = ON, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]