Index: Modules/_io/textio.c
===================================================================
--- Modules/_io/textio.c	(revision 73674)
+++ Modules/_io/textio.c	(working copy)
@@ -1213,11 +1213,18 @@
 static int
 _textiowrapper_writeflush(textio *self)
 {
-    PyObject *b, *ret;
+    PyObject *pending, *b, *ret;
 
     if (self->pending_bytes == NULL)
         return 0;
-    b = _PyBytes_Join(_PyIO_empty_bytes, self->pending_bytes);
+
+    pending = self->pending_bytes;
+    Py_INCREF(pending);
+    self->pending_bytes_count = 0;
+    Py_CLEAR(self->pending_bytes);
+
+    b = _PyBytes_Join(_PyIO_empty_bytes, pending);
+    Py_DECREF(pending);
     if (b == NULL)
         return -1;
     ret = PyObject_CallMethodObjArgs(self->buffer,
@@ -1226,8 +1233,6 @@
     if (ret == NULL)
         return -1;
     Py_DECREF(ret);
-    Py_CLEAR(self->pending_bytes);
-    self->pending_bytes_count = 0;
     return 0;
 }
 
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(revision 73674)
+++ Lib/test/test_io.py	(working copy)
@@ -2031,6 +2031,26 @@
             self.assertEqual(f.errors, "replace")
 
 
+    def test_threads_write(self):
+        # Issue6750: concurrent writes could duplicate data
+        event = threading.Event()
+        with self.open(support.TESTFN, "w", buffering=1) as f:
+            def run(n):
+                text = "Thread%03d\n" % n
+                event.wait()
+                f.write(text)
+            threads = [threading.Thread(target=lambda n=x: run(n)) for x in range(20)]
+            for t in threads:
+                t.start()
+            time.sleep(0.02)
+            event.set()
+            for t in threads:
+                t.join()
+        with self.open(support.TESTFN) as f:
+            content = f.read()
+            for n in range(20):
+                self.assertEquals(content.count("Thread%03d\n" % n), 1)
+
 class CTextIOWrapperTest(TextIOWrapperTest):
 
     def test_initialization(self):