removing dupes from a bag while saving first occurrence

[Tim Chan]

If I have a bag and would like to remove dupes, while saving the first
occurrence, is this possible?

For example, for the following bag:

(group_1,{(2012-12-15,a),(2012-12-17,a),(2012-12-23,c)})

I would like my result to be the following:

(group_1,{(2012-12-15,a),(2012-12-23,c)})