Add clarification on name collision

Author: savannahostrowski

Doc/library/argparse.rst

@@ -1369,6 +1369,10 @@ behavior::
    >>> parser.parse_args('--foo XXX'.split())
    Namespace(bar='XXX')
 
+Multiple arguments may share the same ``dest``.  By default the value from the
+last such argument given on the command line wins; use ``action='append'`` to
+collect values from all of them into a list instead.
+
 .. versionchanged:: 3.15
    Single-dash long option now takes precedence over short options.