At the moment with MongoDb.Drive .Aggregate().Group(...) is impossible to use the $AddToSet operator. Looking at the code it seems that in the Group Translator is Hardcoded the $push operator.
I see that in 2015 that problem was solved using HashSet, but in the 3.1.0 is translated in 2 steps:
{{

followed by { $setUnion }}}
That can generate an out of memory on the $push operation (with a lot of non unique records) that is not generated with the $addToSet.

I think that the best way to solve that is to create a IEnumerable<T> extension called AddToSet(...) that is allowed by MongoDb translator.

Example:

^{public static IEnumerable<TResult> AddToSet<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector)
{{{}}
    return source.Select(selector).Distinct();
}}

that is implemented like:

^{.Group(
    g => g.ImmobileCliente,
    grp => new MyObj()
    {{{}}
        MyProp = grp.AddToSet(i => i.MyProp).ToList(),
    }
)}