fix issue #731: AddDynamicParameters repeated if paramtype is DbStiring should work.

Dapper.Tests/ParameterTests.cs

@@ -1020,6 +1020,18 @@ public void Test_AddDynamicParametersRepeatedShouldWork()
             i.IsEqualTo(123);
         }
 
+        [Fact]
+        public void Test_AddDynamicParametersRepeatedIfParamTypeIsDbStiringShouldWork()
+        {
+            var foo = new DbString() { Value = "123" };
+
+            var args = new DynamicParameters();
+            args.AddDynamicParams(new { Foo = foo });
+            args.AddDynamicParams(new { Foo = foo });
+            int i = connection.Query<int>("select @Foo", args).Single();
+            i.IsEqualTo(123);
+        }
+
         [Fact]
         public void AllowIDictionaryParameters()
         {

Dapper/DbString.cs

@@ -55,8 +55,18 @@ public void AddParameter(IDbCommand command, string name)
             {
                 throw new InvalidOperationException("If specifying IsFixedLength,  a Length must also be specified");
             }
-            var param = command.CreateParameter();
-            param.ParameterName = name;
+            bool add = !command.Parameters.Contains(name);
+            IDbDataParameter param;
+            if (add)
+            {
+                param = command.CreateParameter();
+                param.ParameterName = name;
+
+            }
+            else
+            {
+                param = (IDbDataParameter)command.Parameters[name];
+            }
 #pragma warning disable 0618
             param.Value = SqlMapper.SanitizeParameterValue(Value);
 #pragma warning restore 0618
@@ -69,7 +79,10 @@ public void AddParameter(IDbCommand command, string name)
                 param.Size = Length;
             }
             param.DbType = IsAnsi ? (IsFixedLength ? DbType.AnsiStringFixedLength : DbType.AnsiString) : (IsFixedLength ? DbType.StringFixedLength : DbType.String);
-            command.Parameters.Add(param);
+            if (add)
+            {
+                command.Parameters.Add(param);
+            }
         }
     }
 }