diff --git a/src/calibre/ebooks/oeb/polish/images.py b/src/calibre/ebooks/oeb/polish/images.py
index 1f31722d4f..c1165850cb 100644
--- a/src/calibre/ebooks/oeb/polish/images.py
+++ b/src/calibre/ebooks/oeb/polish/images.py
@@ -16,9 +16,9 @@ class Worker(Thread):
 
     daemon = True
 
-    def __init__(self, abort, name, queue, results, container, jpeg_quality, progress_callback):
+    def __init__(self, abort, name, queue, results, jpeg_quality, progress_callback):
         Thread.__init__(self, name=name)
-        self.queue, self.results, self.container = queue, results, container
+        self.queue, self.results = queue, results
         self.progress_callback = progress_callback
         self.jpeg_quality = jpeg_quality
         self.abort = abort
@@ -27,11 +27,11 @@ class Worker(Thread):
     def run(self):
         while not self.abort.is_set():
             try:
-                name = self.queue.get_nowait()
+                name, path, mt = self.queue.get_nowait()
             except Empty:
                 break
             try:
-                self.compress(name)
+                self.compress(name, path, mt)
             except Exception:
                 import traceback
                 self.results[name] = (False, traceback.format_exc())
@@ -43,16 +43,14 @@ class Worker(Thread):
                     traceback.print_exc()
                 self.queue.task_done()
 
-    def compress(self, name):
+    def compress(self, name, path, mime_type):
         from calibre.utils.img import optimize_png, optimize_jpeg, encode_jpeg
-        mt = self.container.mime_map[name]
-        if 'png' in mt:
+        if 'png' in mime_type:
             func = optimize_png
         elif self.jpeg_quality is None:
             func = optimize_jpeg
         else:
             func = partial(encode_jpeg, quality=self.jpeg_quality)
-        path = self.container.get_file_path_for_processing(name)
         before = os.path.getsize(path)
         with lopen(path, 'rb') as f:
             old_data = f.read()
@@ -80,17 +78,24 @@ def compress_images(container, report=None, names=None, jpeg_quality=None, progr
     results = {}
     queue = Queue()
     abort = Event()
-    for name in images:
-        queue.put(name)
+    seen = set()
+    num_to_process = 0
+    for name in sorted(images):
+        path = os.path.normcase(os.path.abspath(container.get_file_path_for_processing(name)))
+        if path not in seen:
+            num_to_process += 1
+            queue.put((name, path, container.mime_map[name]))
+            seen.add(path)
 
     def pc(name):
-        keep_going = progress_callback(len(results), len(images), name)
+        keep_going = progress_callback(len(results), num_to_process, name)
         if not keep_going:
             abort.set()
-    progress_callback(0, len(images), '')
-    [Worker(abort, 'CompressImage%d' % i, queue, results, container, jpeg_quality, pc) for i in range(min(detect_ncpus(), len(images)))]
+    progress_callback(0, num_to_process, '')
+    [Worker(abort, 'CompressImage%d' % i, queue, results, jpeg_quality, pc) for i in range(min(detect_ncpus(), num_to_process))]
     queue.join()
     before_total = after_total = 0
+    processed_num = 0
     changed = False
     for name, (ok, res) in iteritems(results):
         name = force_unicode(name, filesystem_encoding)
@@ -98,6 +103,7 @@ def compress_images(container, report=None, names=None, jpeg_quality=None, progr
             before, after = res
             if before != after:
                 changed = True
+                processed_num += 1
             before_total += before
             after_total += after
             if report:
@@ -112,8 +118,8 @@ def compress_images(container, report=None, names=None, jpeg_quality=None, progr
     if report:
         if changed:
             report('')
-            report(_('Total image filesize reduced from {0} to {1} [{2:.1%} reduction]').format(
-                human_readable(before_total), human_readable(after_total), (before_total - after_total)/before_total))
+            report(_('Total image filesize reduced from {0} to {1} [{2:.1%} reduction, {3} images changed]').format(
+                human_readable(before_total), human_readable(after_total), (before_total - after_total)/before_total, processed_num))
         else:
             report(_('Images are already fully optimized'))
     return changed, results