Do not add "-- " if already present in .sig file. Attempt 2. Bug 30208. r=bratell, sr=sspitzer.

mailnews/compose/src/nsMsgCompose.cpp

@@ -2626,7 +2626,6 @@ nsMsgCompose::ProcessSignature(nsIMsgIdentity *identity, nsString *aMsgBody)
   // are doing plain text compose, we should insert some sort of message
   // saying "Image Signature Omitted" or something.
   //
-  nsAutoString  urlStr;
   nsXPIDLCString sigNativePath;
   PRBool        useSigFile = PR_FALSE;
   PRBool        htmlSig = PR_FALSE;
@@ -2747,16 +2746,21 @@ nsMsgCompose::ProcessSignature(nsIMsgIdentity *identity, nsString *aMsgBody)
     else
       sigOutput.AppendWithConversion(CRLF);
 
-    sigOutput.AppendWithConversion(dashes);
-
-    if ( (!m_composeHTML) || ((m_composeHTML) && (!htmlSig)) )
-      sigOutput.AppendWithConversion(CRLF);
-    else if (m_composeHTML)
-      sigOutput.AppendWithConversion(htmlBreak);
+    const nsAString& firstFourChars = Substring(sigData, 0, 4);
+    
+    if (!(firstFourChars.Equals(NS_LITERAL_STRING("-- \n")) ||
+          firstFourChars.Equals(NS_LITERAL_STRING("-- \r"))))
+    {
+      sigOutput.AppendWithConversion(dashes);
+    
+      if (!m_composeHTML || !htmlSig)
+        sigOutput.AppendWithConversion(CRLF);
+      else if (m_composeHTML)
+        sigOutput.AppendWithConversion(htmlBreak);
+    }
 
     sigOutput.Append(sigData);
-         //DELETEME: I18N: Converting down (2byte->1byte) OK?
-
+    
     if (m_composeHTML)
     {
       if (htmlSig)