delete lines that contain duplicated column items

[Jeff Zhang]

I have a simple txt file, like:
...
a a
aa a
b b
ba b
...

I want to just keep the lines that first appeared in column 2 and delete the
follow lines that contain duplicated ones in column 2.
then it will like:
...
a a
b b
...

I've tried `uniq -f1`  but it didn't work.