colelction was modified enumeration may not execute problem while mailing.

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C#

Hello,
I have a windows service it reads the defined rules and after it sends emails to the recipients. on the emails that it send, on the "To:" part of the email the recipients emails are repeated several times. I needed them to be shown once.Also When I debug the code I receive an exception such as "collection was modified enumeration may not execute".But I am not adding or deleting something from the collection. The code is;

foreach (Rules rl in global_rules)
                {
                    if (rl._ratingtype == "Equal to")
                    {
                        if (Convert.ToInt32(rating) == Convert.ToInt32(rl._rating))
                        {
                            message.To.Add(new System.Net.Mail.MailAddress(rl._mailto));
                        }
                    }
                    else if (rl._ratingtype == "Greater than and equal to")
                    {
                        if (Convert.ToInt32(rating) >= Convert.ToInt32(rl._rating))
                        {
                            message.To.Add(new System.Net.Mail.MailAddress(rl._mailto));
                        }
                    }
                    else if (rl._ratingtype == "Less than and equal to")
                    {
                        if (Convert.ToInt32(rating) <= Convert.ToInt32(rl._rating))
                        {
                            message.To.Add(new System.Net.Mail.MailAddress(rl._mailto));
                        }
                    }
                }