diff --git a/paramiko/sftp_client.py b/paramiko/sftp_client.py
index f0f8fcb..3a65a68 100644
--- a/paramiko/sftp_client.py
+++ b/paramiko/sftp_client.py
@@ -185,7 +185,7 @@ class SFTPClient (BaseSFTP):
         if ('w' in mode):
             imode |= SFTP_FLAG_CREATE | SFTP_FLAG_TRUNC
         if ('a' in mode):
-            imode |= SFTP_FLAG_APPEND
+            imode |= SFTP_FLAG_APPEND | SFTP_FLAG_CREATE | SFTP_FLAG_WRITE
         attrblock = SFTPAttributes()
         t, msg = self._request(CMD_OPEN, filename, imode, attrblock)
         if t != CMD_HANDLE:
diff --git a/paramiko/sftp_file.py b/paramiko/sftp_file.py
index 6b8e935..854ccf4 100644
--- a/paramiko/sftp_file.py
+++ b/paramiko/sftp_file.py
@@ -45,6 +45,15 @@ class SFTPFile (BufferedFile):
         self.close()
         
     def close(self):
+        # We allow double-close without signaling an error, because real
+        # Python file objects do.  However, we must protect against actually
+        # sending multiple CMD_CLOSE packets, because after we close our
+        # handle, the same handle may be re-allocated by the server, and we
+        # may end up mysteriously closing some random other file.  (This is
+        # especially important because we unconditionally call close() from
+        # __del__.)
+        if self._closed:
+            return
         BufferedFile.close(self)
         try:
             self.sftp._request(CMD_CLOSE, self.handle)