diff -r 08dc10ef9ab1 -r ecbfec2da8a2 server/schemaserial.py
--- a/server/schemaserial.py	Fri Sep 19 13:08:26 2014 +0200
+++ b/server/schemaserial.py	Wed Nov 26 17:58:23 2014 +0100
@@ -421,7 +421,14 @@
 
 def uniquetogether2rqls(eschema):
     rql_args = []
+    # robustness against duplicated CWUniqueTogetherConstraint (pre 3.18)
+    columnset = set()
     for columns in eschema._unique_together:
+        if columns in columnset:
+            print ('schemaserial: skipping duplicate unique together %r %r' %
+                   (eschema.type, columns))
+            continue
+        columnset.add(columns)
         rql, args = _uniquetogether2rql(eschema, columns)
         args['name'] = y2sql.unique_index_name(eschema, columns)
         rql_args.append((rql, args))