diff --git a/src/calibre/ebooks/oeb/polish/container.py b/src/calibre/ebooks/oeb/polish/container.py
index b1a873782c..21c7b445db 100644
--- a/src/calibre/ebooks/oeb/polish/container.py
+++ b/src/calibre/ebooks/oeb/polish/container.py
@@ -232,6 +232,12 @@ class Container(object):  # {{{
         self.dirty(self.opf_name)
         return item_id
 
+    def manifest_has_name(self, name):
+        ''' Return True if the manifest has an entry corresponding to name '''
+        href = self.name_to_href(name, self.opf_name)
+        all_hrefs = {x.get('href') for x in self.opf_xpath('//opf:manifest/opf:item[@href]')}
+        return href in all_hrefs
+
     def add_file(self, name, data, media_type=None, spine_index=None):
         ''' Add a file to this container. Entries for the file are
         automatically created in the OPF manifest and spine
diff --git a/src/calibre/gui2/tweak_book/boss.py b/src/calibre/gui2/tweak_book/boss.py
index 8e029b071e..fb50e79854 100644
--- a/src/calibre/gui2/tweak_book/boss.py
+++ b/src/calibre/gui2/tweak_book/boss.py
@@ -432,7 +432,7 @@ class Boss(QObject):
             c = current_container()
             for path, name in files.iteritems():
                 i = 0
-                while c.exists(name):
+                while c.exists(name) or c.manifest_has_name(name):
                     i += 1
                     name, ext = name.rpartition('.')[0::2]
                     name = '%s_%d.%s' % (name, i, ext)