Question: Trouble with unions of sets of vectors

I've just gotten started with maple (2 days ago). I needed to construct a set whose elements are vectors. During the generation of these vectors, there turn out to be duplicates and I was hoping that the set union operation will get rid of these. Instead, I end up with a set consisting of duplicated vectors. Here is a very simplified case that my problem boils down to: with(LinearAlgebra); a:=UnitVector(1,2); b:=UnitVector(1,2); #a and b are clearly equal. s:={a,b}; #s should be just the singleton set. But it gives s as being a 2 element set with this vector repeated. Apparently maple considers a and b to be two different elements ? Any help with this will be highly appreciated ! Thanks Viswanath.
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