diff --git a/src/calibre/ebooks/mobi/reader.py b/src/calibre/ebooks/mobi/reader.py
index 2033ff11f5..09a970ef60 100644
--- a/src/calibre/ebooks/mobi/reader.py
+++ b/src/calibre/ebooks/mobi/reader.py
@@ -79,7 +79,9 @@ class EXTHHeader(object):
         elif id == 105:
             if not self.mi.tags:
                 self.mi.tags = []
-            self.mi.tags.append(content.decode(codec, 'ignore'))
+            self.mi.tags.extend([x.strip() for x in content.decode(codec,
+                'ignore').split(';')])
+            self.mi.tags = list(set(self.mi.tags))
         elif id == 106:
             try:
                 self.mi.publish_date = datetime.datetime.strptime(