How to create unique index for month and year column?
Assuming you have a column of type DATE or DATETIME2 (or DATETIME), you can define two computed, persisted columns for year and month and put an unique index on those:

ALTER TABLE dbo.YourTable
ADD DateMonth AS MONTH(YourDateColumn) PERSISTED

ALTER TABLE dbo.YourTable
ADD DateYear AS YEAR(YourDateColumn) PERSISTED

CREATE UNIQUE INDEX UIX_YourTable_MonthYear ON dbo.YourTable(DateMonth, DateYear)