Remove duplication.
diff --git a/Doc/library/mailbox.rst b/Doc/library/mailbox.rst
index ff8cfea..83a590e 100644
--- a/Doc/library/mailbox.rst
+++ b/Doc/library/mailbox.rst
@@ -780,7 +780,7 @@
There is no requirement that :class:`Message` instances be used to represent
messages retrieved using :class:`Mailbox` instances. In some situations, the
time and memory required to generate :class:`Message` representations might
- not not acceptable. For such situations, :class:`Mailbox` instances also
+ not be acceptable. For such situations, :class:`Mailbox` instances also
offer string and file-like representations, and a custom message factory may
be specified when a :class:`Mailbox` instance is initialized.