Union
Union[expr1, expr2, ...]
returns a sorted union of the items in the expressions.
Attributes[Union] := {Protected}
Simple examples
In[1]:= Union[{b, a, a, b, a}] Out[1]= {a, b}
In[2]:= Union[{b, a, a, b, a}, {y, z}] Out[2]= {a, b, y, z}
In[3]:= Union[foo[b, a, a, b, a], foo[y, z]] Out[3]= foo[a, b, y, z]