Possible workaround:
You can subscribe to the PreviewTextInput event of the control and change the position of the caret if the typed char is the same as the current one.

private void RadMaskedTextInput_PreviewTextInput(object sender, TextCompositionEventArgs e)
        {
            var input = sender as RadMaskedTextInput;
            var currentPosition = input.SelectionStart;

            if (currentPosition != input.Text.Length)
            {
                var currentChar = input.Text[currentPosition];

                if (currentChar != input.Placeholder && currentChar.ToString() == e.Text)
                {
                    input.SelectionStart++;
                }
            }
        }



Available in R3 2016 Release