Min spanning tree is all about edges I think so why you are considering vertex? From my point of view in this graph we have two spanning tree of equal weight as you mentioned. :

1st one : edges :{(a,b),(b,c),(c,i),(i,d)} total weight : 21

2nd one : edges :{(a,b),(a,d),(d,i),(i,c)} total weight : 21

1st one : edges :{(a,b),(b,c),(c,i),(i,d)} total weight : 21

2nd one : edges :{(a,b),(a,d),(d,i),(i,c)} total weight : 21