Is it possible for unique column to contain multiple null values SQL Server 2008
You can use a Filtered Index to remove the NULL values from consideration regarding uniqueness. You would first drop the existing UNIQUE CONSTRAINT and then replace it with a UNIQUE INDEX. These two objects are essentially the same given that a UNIQUE CONSTRAINT actually exists via a UNIQUE INDEX (the documentation for creating a column constraint states that UNIQUE is a "constraint that provides entity integrity for a specified column or columns by using a unique index.").

CREATE UNIQUE NONCLUSTERED INDEX index_name
ON SchemaName.TableA (CIN ASC)
WHERE [CIN] IS NOT NULL;