Make sure saved oids don't get duped.

This is added insurance on
https://bugs.launchpad.net/zodb/+bug/665452.
In particular, because we saved dups, the damage persisted, requiring
a process restart to clear.

src/ZODB/DB.py

@@ -475,7 +475,7 @@ class DB(object):
         databases[database_name] = self
         self.xrefs = xrefs
 
-        self._saved_oids = []
+        self._saved_oids = set()
         self._max_saved_oids = max_saved_oids
         self.large_record_size = large_record_size
 
@@ -959,8 +959,10 @@ class DB(object):
 
 
     def save_oid(self, oid):
+        if oid in self._saved_oids:
+            raise ValueError("Duplicate saved object ids.")
         if len(self._saved_oids) < self._max_saved_oids:
-            self._saved_oids.append(oid)
+            self._saved_oids.add(oid)
 
     def new_oid(self):
         if self._saved_oids:

src/ZODB/tests/testDB.py

@@ -345,6 +345,29 @@ def minimally_test_connection_timeout():
 
     """
 
+def saving_oid_multiple_times_doesnt_cause_dups():
+    r"""Duplicate saves of an oid shouldn't happen unless there's a bug elsewhere
+
+    But saving dups makes matters worse, because it dooms the process,
+    not just the transaction.
+
+    >>> db = ZODB.DB(None)
+    >>> oid = db.new_oid()
+    >>> db.save_oid(oid)
+    >>> db.new_oid() is oid
+    True
+    >>> db.save_oid(oid)
+    >>> db.save_oid(oid)
+    Traceback (most recent call last):
+    ...
+    ValueError: Duplicate saved object ids.
+
+    >>> db.new_oid() is oid
+    True
+    >>> db.new_oid()
+    '\x00\x00\x00\x00\x00\x00\x00\x02'
+
+    """
 
 def test_suite():
     s = unittest.makeSuite(DBTests)