--- a/langtools/src/share/classes/com/sun/tools/javac/main/JavaCompiler.java	Tue Sep 07 15:49:48 2010 -0700
+++ b/langtools/src/share/classes/com/sun/tools/javac/main/JavaCompiler.java	Thu Sep 09 09:42:45 2010 +0530
@@ -872,8 +872,13 @@
 
         //parse all files
         ListBuffer<JCCompilationUnit> trees = lb();
-        for (JavaFileObject fileObject : fileObjects)
-            trees.append(parse(fileObject));
+        Set<JavaFileObject> filesSoFar = new HashSet<JavaFileObject>();
+        for (JavaFileObject fileObject : fileObjects) {
+            if (!filesSoFar.contains(fileObject)) {
+                filesSoFar.add(fileObject);
+                trees.append(parse(fileObject));
+            }
+        }
         return trees.toList();
     }
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/langtools/test/tools/javac/T6900149.java	Thu Sep 09 09:42:45 2010 +0530
@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 6900149
+ * @summary IllegalStateException when compiling same files and DiagnosticListener is set
+ */
+
+import java.io.*;
+import java.util.*;
+import javax.tools.*;
+import javax.tools.JavaCompiler.CompilationTask;
+
+public class T6900149 {
+    public static void main(String[] args) throws IOException {
+        DiagnosticCollector<JavaFileObject> diag =
+                new DiagnosticCollector<JavaFileObject>();
+        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
+        StandardJavaFileManager fm =
+                compiler.getStandardFileManager(null, null, null);
+        File emptyFile = File.createTempFile("Empty", ".java");
+        File[] files = new File[] { emptyFile, emptyFile };
+        CompilationTask task = compiler.getTask(null, fm, diag,
+                null, null, fm.getJavaFileObjects(files));
+        if (! task.call()) {
+            throw new AssertionError("compilation failed");
+        }
+    }
+}