minor typo

Author: puentesarrin

source/core/replication.txt

@@ -169,7 +169,7 @@ In MongoDB, all read operations issued to the primary of a
 replica set are :term:`consistent <strict consistency>` with the last
 write operation.
 
-If clients configure the :term:`read preference` to permit allow secondary reads,
+If clients configure the :term:`read preference` to permit secondary reads,
 read operations cannot return from :term:`secondary` members that have not
 replicated more recent updates or operations. In these situations the
 query results may reflect a previous state.