Unique Non-Key field

[Jamie Cantrell]

I have a non-key field in a table that I need to enforce unique values in. Adding this field as part of the primary key field will not necessarily force unique values due to the fact the existing primary key field contains unique values.

For example:
1,a
2,a
3,a

Each would be considered unique primary keys, but I need to force unique values in the second column as well as the first. Please help