Ensure domains are canonicalised

So, each domain owner only has one entry in the file (but can still
have multiple domains).

tests/app/test_utils.py

@@ -1,4 +1,4 @@
-from collections import OrderedDict
+from collections import Counter, OrderedDict
 from csv import DictReader
 from io import StringIO
 from pathlib import Path
@@ -415,6 +415,18 @@ def test_validate_government_domain_data():
         }
 
 
+def test_domain_data_is_canonicalized():
+    for owner, count in Counter(
+        AgreementInfo(domain).owner
+        for domain in AgreementInfo.domains.keys()
+        if AgreementInfo(domain).is_canonical
+    ).most_common():
+        if count > 1:
+            raise ValueError(
+                '{} entries in domains.yml for {}'.format(count, owner)
+            )
+
+
 def test_validate_email_domain_data():
 
     for domain in GovernmentEmailDomain.domains.keys():