Ignore duplicate project-template definitions

Let the first one win, so that they can't be updated in other branches or
repos.

Change-Id: I5d0c2d428b0714935949d1e0350ab98ef2969293

tests/fixtures/config/in-repo/git/common-config/playbooks/template-job.yaml

@@ -0,0 +1,2 @@
+- hosts: all
+  tasks: []

tests/fixtures/config/in-repo/git/common-config/zuul.yaml

@@ -76,6 +76,15 @@
 - job:
     name: common-config-test
 
+- job:
+    name: template-job
+
+- project-template:
+    name: common-config-template
+    check:
+      jobs:
+        - template-job
+
 - project:
     name: common-config
     check: