Added tests to complement documentation of pseudo-positional parameters

Dapper.Tests/Tests.cs

@@ -3315,6 +3315,38 @@ public void Issue569_SO38527197_PseudoPositionalParameters_In()
             }
         }
 
+        [Fact]
+        public void PseudoPositional_CanUseVariable()
+        {
+            using (var connection = ConnectViaOledb())
+            {
+                int id = 42;
+                var row = connection.QuerySingle("declare @id int = ?id?; select @id as [A], @id as [B];", new { id });
+                int a = (int)row.A;
+                int b = (int)row.B;
+                a.IsEqualTo(42);
+                b.IsEqualTo(42);
+            }
+        }
+        [Fact]
+        public void PseudoPositional_CannotUseParameterMultipleTimes()
+        {
+
+            using (var connection = ConnectViaOledb())
+            {
+                try
+                {
+                    int id = 42;
+                    var row = connection.QuerySingle("select ?id? as [A], ?id? as [B];", new { id });
+                    Assert.Fail();
+                }
+                catch (InvalidOperationException ex) when (ex.Message == "When passing parameters by position, each parameter can only be referenced once")
+                {
+                    // that's a win
+                }
+            }
+        }
+
         [Fact]
         public void PseudoPositionalParameters_ExecSingle()
         {