В случае любого количества слэсов вы можете использовать Regex.Replace
метод:
public string ReplaceSlashes(string source)
{
var pattern = @"\\+";
return Regex.Replace(source, pattern, @"\");
}
результаты:
ReplaceSlashes(@"SOUTH MOTOR COMPANY C\O CALDWELL AND KERR"); // SOUTH MOTOR COMPANY C\O CALDWELL AND KERR
ReplaceSlashes(@"SOUTH MOTOR COMPANY C\\O CALDWELL AND KERR"); // SOUTH MOTOR COMPANY C\O CALDWELL AND KERR
ReplaceSlashes(@"SOUTH MOTOR COMPANY C\\\\\\\\O CALDWELL AND KERR"); // SOUTH MOTOR COMPANY C\O CALDWELL AND KERR