Automatically remove duplicate recipients

[Jericho]

When an email is sent with duplicate recipient, SendGrid does not throw any exception (which gives you the impression the email was sent) but it does not actually send the email. Therefore StrongGrid should automatically remove duplicate recipients.

This is discussed in several issues in SendGrid's csharp client repository:

• SendGridMessage creates BadRequest when using both AddCc and AddBcc sendgrid/sendgrid-csharp#400
• Emails are not sent when using both a TO and CC Address sendgrid/sendgrid-csharp#409
• Impement IEquatable on EmailAddress to Filter Duplicates sendgrid/sendgrid-csharp#425
• Adding BCC to Sengrid 9.1.1 gives Bad Request sendgrid/sendgrid-csharp#448
• Email not sent if To address is the same as Bcc address sendgrid/sendgrid-csharp#451
• Email Not Sent When Non-Unique From, To, and CC Used sendgrid/sendgrid-csharp#503
• Sending different mail content to same recipient does not work well sendgrid/sendgrid-csharp#520
• BadRequest if same email address is in the To & BCC fields. sendgrid/sendgrid-csharp#678
• No email is sent when recipient email is duplicated sendgrid/sendgrid-csharp#691