Do not repeat stack definitions.

software/erp5/software.cfg

 [buildout]
-extensions =
-  slapos.rebootstrap
-  slapos.tool.networkcache
-  slapos.zcbworkarounds
-  mr.developer
-
-find-links =
-    http://www.nexedi.org/static/packages/source/slapos.buildout/
-    http://dist.repoze.org
-    http://www.nexedi.org/static/packages/source/
-
 extends =
   ../../stack/erp5.cfg