Duplicate directive registration allowed

[Malthe Borch]

In zope/configuration/config.py:register it seems that it isn't being
asserted that no previous registration of a directive with the same
name has happened.

This might be desirable (not sure why) and currently happens for these
directives:

('http://namespaces.zope.org/meta', u'directive')
('http://namespaces.zope.org/meta', u'groupingDirective')
('http://namespaces.zope.org/meta', u'complexDirective')
('http://namespaces.zope.org/zope', u'securityPolicy')

So for instance, if Five registers the <browser:viewlet> then there's
nothing to prevent another package from including zope.viewlet's
meta.zcml which will then override that directive, breaking viewlet
support in Zope 2.

[Tres Seaver (tseaver)]

Please follow up on Github:

 https://github.com/zopefoundation/zope.configuration/issues/7