+ ``--filter-by=``\ (**nameaddr**\ \|\ **name** \|\ **addr**\ \|\ **addrfold**\ \|\ **nameaddrfold**\)
+
+ Controls how to filter out duplicate addresses. The filtering
+ algorithm receives a sequence of email addresses and outputs
+ the same sequence without the addresses that are considered a
+ duplicate of a previously output address. What is considered a
+ duplicate depends on how the two addresses are compared:
+
+ **nameaddr** means that both name and address parts are
+ compared in case-sensitive manner. Therefore, all same looking
+ addresses strings are considered duplicate. This is the
+ default.
+
+ **name** means that only the name part is compared (in
+ case-sensitive manner). For example, the addresses "John Doe
+ <me@example.com>" and "John Doe <john@doe.name>" will be
+ considered duplicate.
+
+ **addr** means that only the address part is compared (in
+ case-sensitive manner). For example, the addresses "John Doe
+ <john@example.com>" and "Dr. John Doe <john@example.com>" will
+ be considered duplicate.
+
+ **addrfold** is like **addr**, but comparison is done in
+ canse-insensitive manner. For example, the addresses "John Doe
+ <john@example.com>" and "Dr. John Doe <JOHN@EXAMPLE.COM>" will
+ be considered duplicate.
+
+ **nameaddrfold** is like **nameaddr**, but address comparison
+ is done in canse-insensitive manner. For example, the
+ addresses "John Doe <john@example.com>" and "John Doe
+ <JOHN@EXAMPLE.COM>" will be considered duplicate.
+