Duplicate parameters can be defined

[bgrainger]

using (var cmd = new MySqlCommand("SELECT @foo;", connection))
{
	cmd.Parameters.AddWithValue("foo", 1);
	cmd.Parameters.AddWithValue("@foo", 2);
	cmd.ExecuteScalar();
}

Expected: MySqlException: Parameter '@foo' has already been defined.

Actual: no exception; ExecuteScalar returns 2.

(This happens with any combination of foo and @foo as the parameter names.)

May be fixed as part of #589.

[bgrainger]

Fixed in 0.48.0.