diff --git a/tests/fixtures/layouts/multiple-templates.yaml b/tests/fixtures/layouts/multiple-templates.yaml
new file mode 100644
index 0000000..7272cad
--- /dev/null
+++ b/tests/fixtures/layouts/multiple-templates.yaml
@@ -0,0 +1,44 @@
+- pipeline:
+    name: check
+    manager: independent
+    trigger:
+      gerrit:
+        - event: patchset-created
+    success:
+      gerrit:
+        Verified: 1
+    failure:
+      gerrit:
+        Verified: -1
+
+- job:
+    name: base
+    parent: null
+
+- job:
+    name: py27
+
+- project-template:
+    name: python-jobs
+    check:
+      jobs:
+        - py27
+
+- project-template:
+    name: python-trusty-jobs
+    check:
+      jobs:
+        - py27:
+            tags:
+              - trusty
+
+- project:
+    name: org/project1
+    templates:
+      - python-jobs
+      - python-trusty-jobs
+
+- project:
+    name: org/project2
+    templates:
+      - python-jobs
diff --git a/tests/unit/test_scheduler.py b/tests/unit/test_scheduler.py
index 2dcd9bf..f9a83ab 100755
--- a/tests/unit/test_scheduler.py
+++ b/tests/unit/test_scheduler.py
@@ -4613,6 +4613,17 @@
         self.assertIn('project-test1 : SKIPPED', A.messages[1])
         self.assertIn('project-test2 : SKIPPED', A.messages[1])
 
+    @simple_layout('layouts/multiple-templates.yaml')
+    def test_multiple_project_templates(self):
+        # Test that applying multiple project templates to a project
+        # doesn't alter them when used for a second project.
+        A = self.fake_gerrit.addFakeChange('org/project2', 'master', 'A')
+        self.fake_gerrit.addEvent(A.getPatchsetCreatedEvent(1))
+        self.waitUntilSettled()
+
+        build = self.getJobFromHistory('py27')
+        self.assertEqual(build.parameters['zuul']['jobtags'], [])
+
 
 class TestExecutor(ZuulTestCase):
     tenant_config_file = 'config/single-tenant/main.yaml'
diff --git a/zuul/model.py b/zuul/model.py
index 4c5a51f..5c95cd5 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -1030,7 +1030,9 @@
             if jobname in self.jobs:
                 self.jobs[jobname].extend(jobs)
             else:
-                self.jobs[jobname] = jobs
+                # Be sure to make a copy here since this list may be
+                # modified.
+                self.jobs[jobname] = jobs[:]
 
 
 class JobGraph(object):