How to reproduce: simply configure Polish keyboard and press [ALT GR] + [C], the added text is "©ć". The expected behavior would be to only add "ć"

Workaround: Handle the CommandExecuting event:
private void RadRichTextEditor1_CommandExecuting(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
{
    if (e.Command is InsertTextCommand && e.CommandParameter.ToString() == "©")
    {
        e.Cancel = true;
    }
}