In #58 I tried to fix this issue:

In WMA

In[1]:= F[a, Unevaluated[a]]
Out[1]:=F[a, Unevaluated[a]]

In Mathics

In[1]:= F[a, Unevaluated[a]]
Out[1]:=F[Unevaluated[a], Unevaluated[a]]

This happens because as now Symbols are singletons, the old mechanism of setting a property does not work anymore. In #58, I tried to fix this using a table. However, this does not work in general, if F has the attributes Orderless or Flat, because the order is lost.