remove duplicate(non-unique) paired values in stata, urgent : I'm working with an edge list in STATA, of the type: 

var1 var2
 a    1
 a    2
 a    3
 b    1
 b    2
 1    a
 2    b

I want to remove non-unique pairs such as 1a and 2b (which are same as a1 and b2 for me). How can I go about this? 