This code:

var s1 = session.Query.All<MyEntity>().Select(a => new { T1 = a.Text, T2 = a.Text });
var s2 = session.Query.All<MyEntity>().Select(a => new { T1 = a.Text, T2 = a.Text });

var s3 = s1.Concat(s2);
var sss = s3.ToArray();

same bug for this:

        var q1 = from q in Query.All<RegEntity>() select new { q.Id, q };
        var q2 = from q in Query.All<RegEntity>() select new { q.Id, q };
        var un = q1.Concat(q2);
        var un2 = q1.Union(q2);

Raises exception: System.ArgumentException occurred

  Message=An item with the same key has already been added.
  Source=mscorlib
  StackTrace:
       at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
       at Xtensive.Storage.Rse.ColumnCollection.Initialize() in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Storage\Rse\ColumnCollection.cs:line 43
  InnerException: