Index: contrib/jcr-server/server/src/java/org/apache/jackrabbit/server/io/AddMixinCommand.java
===================================================================
--- contrib/jcr-server/server/src/java/org/apache/jackrabbit/server/io/AddMixinCommand.java	(revision 170644)
+++ contrib/jcr-server/server/src/java/org/apache/jackrabbit/server/io/AddMixinCommand.java	(working copy)
@@ -21,6 +21,7 @@
 import org.apache.jackrabbit.JcrConstants;
 
 import javax.jcr.Node;
+import javax.jcr.nodetype.NodeType;
 
 /**
  * This Class implements a import command that adds a mixin node type to the
@@ -94,7 +95,18 @@
         if (nodeType == null) {
             throw new IllegalArgumentException("AddMixinCommand needs 'nodeType' attribute.");
         }
-        parentNode.addMixin(nodeType);
+        // check to see if the node already has this type
+        boolean needsType = true;
+        NodeType[] existing = parentNode.getMixinNodeTypes();
+        for (int i=0; i<existing.length; i++) {
+            if (existing[i].getName().equals(nodeType)) {
+                needsType = false;
+                break;
+            }
+        }
+        if (needsType) {
+            parentNode.addMixin(nodeType);
+        }
         return false;
     }
 }