Prevent duplicate month/year form

Prevent duplicate month/year form

The forms in my application are set up to record financial indicators by month and year.

Currently I have a simple drop down box to select the month and a single line field to manually enter the year. 

The problem is that the month and year can easily be duplicated with a separate entry. How would I make the month and year fields exclusive so the same month and year combination can not be duplicated?

Also, I assume a Look Up field would be needed to modify the data for a particular month.