You can use unique to get the elements without repetition (subject to sum being less than 9007199254740992, as mentioned below)
%random data
a=[5 4 5 3 2 6 1 6 1 6];
b=unique(a,'stable')
%using 'stable' to give an idea about the order
%you can use unique without 'stable' option as well
s=sum(b)
sum(unique(a)) and sum(unique(a,'stable')) won't give the same answer.
