Index: src/main/org/apache/tools/ant/taskdefs/Zip.java
===================================================================
--- src/main/org/apache/tools/ant/taskdefs/Zip.java	(revision 384498)
+++ src/main/org/apache/tools/ant/taskdefs/Zip.java	(working copy)
@@ -1130,11 +1130,15 @@
                 resources = selectFileResources(resources);
             }
 
-            newerResources[i] =
-                ResourceUtils.selectOutOfDateSources(this,
+            if(duplicate.equals("overwrite")){
+                newerResources[i] = resources;
+            } else {
+                newerResources[i] =
+                    ResourceUtils.selectOutOfDateSources(this,
                                                      resources,
                                                      myMapper,
                                                      getZipScanner());
+            }
             needsUpdate = needsUpdate || (newerResources[i].length > 0);
 
             if (needsUpdate && !doUpdate) {
@@ -1647,7 +1651,7 @@
          * @see EnumeratedAttribute#getValues()
          */
         public String[] getValues() {
-            return new String[] {"add", "preserve", "fail"};
+            return new String[] {"add", "preserve", "fail", "overwrite"};
         }
     }