Revert "Allow multiple invocations of the same job"

This reverts commit 3aa372722546252f629ad996ce206ece5ab7e582.

There are several more places in Zuul where it is assumed that job
names are unique.  Fixing them all would be complicated and perhaps
not worth the benefit.  Go back to requiring that they be unique for
a project.

Change-Id: I5b8aa33d6aed9cb086f44594ce59947c0190cc11
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 7274342..9649c8e 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -772,8 +772,8 @@
 
 Note that if multiple templates are used for a project and one
 template specifies a job that is also specified in another template,
-or specified in the project itself, those jobs will be duplicated in
-the resulting project configuration.
+or specified in the project itself, the configuration defined by
+either the last template or the project itself will take priority.
 
 logging.conf
 ~~~~~~~~~~~~