removing repetitions from list with same elements but in reverse order

Hi,

I have a list which looks like this.

for example

a=( (1  2) (2  1)  (1  3)  (3  1) ) .  I need a function to transverse this list  as shown

a=( (1 2) (1 3) )  or a = ( (2  1)  (3  1) )

Thanks,

Raghu