Remove duplication.
diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 78f2b4d..56a2a34 100644
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -211,7 +211,7 @@
See also the :ref:`formatspec` section.
-The *field_name* itself begins with an *arg_name* that is either either a number or a
+The *field_name* itself begins with an *arg_name* that is either a number or a
keyword. If it's a number, it refers to a positional argument, and if it's a keyword,
it refers to a named keyword argument. If the numerical arg_names in a format string
are 0, 1, 2, ... in sequence, they can all be omitted (not just some)