diff --git a/fsl/utils/cache.py b/fsl/utils/cache.py
index 6d39d916cac771cd5f8f4917c8bd762c9cd09831..c49a6ff16c0d2e047afde6b3107f45fb2a02b809 100644
--- a/fsl/utils/cache.py
+++ b/fsl/utils/cache.py
@@ -63,7 +63,8 @@ class Cache(object):
                      ``0`` will not expire.
         """
 
-        if len(self.__cache) == self.__maxsize:
+        if len(self.__cache) == self.__maxsize and \
+           key not in self.__cache:
             self.__cache.popitem(last=False)
 
         self.__cache[key] = CacheItem(key, value, expiry)
@@ -105,11 +106,15 @@ class Cache(object):
 
 
     def clear(self):
-        """Remove all items fromthe cache.
-        """
+        """Remove all items from the cache. """
         self.__cache = collections.OrderedDict()
 
 
+    def __len__(self):
+        """Returns the number of items in the cache. """
+        return len(self.__cache)
+
+
     def __parseDefault(self, *args, **kwargs):
         """Used by the :meth:`get` method. Parses the ``default`` argument,
         which may be specified as either a positional or keyword argumnet.