In fact, aliases for programs or mailboxes is not directly supported by Courier's aliasing mechanisms. It's implemented by having the makealiases script automatically create a .courier file, and point the alias address to it. An alias containing only one address does not affect delivery status notification attributes of an E-mail message. An alias with multiple addresses is treated like a private mailing list, as defined by RFC 1894. This has nothing to do with Courier's support for a Qmail-style alias account.
Courier normally tries to eliminate duplicate addresses listed as recipients for the same message.
Some mail servers are not capable of delivering messages with multiple recipients, and will transmit a separate copy of the same message addressed to each recipient. Courier can't do anything in this case. Each copy of the same original text is considered an individual, separate, message. Duplicate elimination can fail in certain rare circumstances, involving exotic features of RFC 822 concerning case sensitivity. Because multiple-recipient aliases are treated like private mailing lists, failure DSNs are turned off, and a bad recipient address is hardly noticed by anyone.