Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 68142)
+++ Lib/io.py	(working copy)
@@ -1692,7 +1692,9 @@
         self._snapshot = None
 
         # Restore the decoder to its state from the safe start point.
-        if self._decoder or dec_flags or chars_to_skip:
+        if cookie == 0 and self._decoder:
+            self._decoder.reset()
+        elif self._decoder or dec_flags or chars_to_skip:
             self._decoder = self._decoder or self._get_decoder()
             self._decoder.setstate((b'', dec_flags))
             self._snapshot = (dec_flags, b'')
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(revision 68142)
+++ Lib/test/test_io.py	(working copy)
@@ -1080,6 +1080,8 @@
             f.write(data)
             f.seek(0)
             self.assertEquals(f.read(), data * 2)
+            f.seek(0)
+            self.assertEquals(f.read(), data * 2)
             self.assertEquals(buf.getvalue(), (data * 2).encode(encoding))
 
     def timingTest(self):