LockSet: allow lists with duplicate values

If a list with a duplicate value is passed to a lockset what the code
now does is to try to acquire the lock twice, generating a
double-acquire exception in the SharedLock code. This is definitely an
issue. In order to solve it we can either forbit double values in a list
or just delete the duplicates. In this patch we go for the latter
solution, removing any duplicate values when creating the acquire_list.

Reviewed-by: imsnah

lib/locking.py

@@ -452,7 +452,7 @@ class LockSet:
       # First we look the locks up on __lockdict. We have no way of being sure
       # they will still be there after, but this makes it a lot faster should
       # just one of them be the already wrong
-      for lname in names:
+      for lname in utils.UniqueSequence(names):
         try:
           lock = self.__lockdict[lname] # raises KeyError if lock is not there
           acquire_list.append((lname, lock))