AMPL error, duplicate number for set

Question

In AMPL, I have a set which should store some similar values. But I have a "duplicate number" error.

Is there any way to do that? What is the simplest method to solve this problem?

The set is:

set A;

data: set A := 1 1 2;

Thanks

Answer 1

Set elements should be unique in AMPL. To store duplicate values use parameter instead:

set S;
param A{S};
data;
param:
 S: A :=  
 1  1
 2  1
 3  2;