Index: jackrabbit-core/src/main/java/org/apache/jackrabbit/core/state/LocalItemStateManager.java
===================================================================
--- jackrabbit-core/src/main/java/org/apache/jackrabbit/core/state/LocalItemStateManager.java	(revision 793243)
+++ jackrabbit-core/src/main/java/org/apache/jackrabbit/core/state/LocalItemStateManager.java	Thu Aug 27 12:20:11 CEST 2009
@@ -407,11 +407,19 @@
             try {
                 local = changeLog.get(created.getId());
                 if (local != null) {
+                    if (local.getOverlayedState() == created) {
-                    // underlying state has been permanently created
-                    local.pull();
-                    local.setStatus(ItemState.STATUS_EXISTING);
-                    cache.cache(local);
+                        // underlying state has been permanently created
+                        local.pull();
+                        local.setStatus(ItemState.STATUS_EXISTING);
+                        cache.cache(local);
+                    } else {
+                        // mid-air collision of concurrent item state creation
+                        // with same id (JCR-2272)
+                        if (local.getStatus() == ItemState.STATUS_NEW) {
+                            local.setStatus(ItemState.STATUS_STALE_MODIFIED);
-                }
+                        }
+                    }
+                }
             } catch (NoSuchItemStateException e) {
                 /* ignore */
             }
Index: jackrabbit-core/src/main/java/org/apache/jackrabbit/core/state/SharedItemStateManager.java
===================================================================
--- jackrabbit-core/src/main/java/org/apache/jackrabbit/core/state/SharedItemStateManager.java	(revision 803470)
+++ jackrabbit-core/src/main/java/org/apache/jackrabbit/core/state/SharedItemStateManager.java	Thu Aug 27 15:38:58 CEST 2009
@@ -20,6 +20,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Set;
+import java.util.Calendar;
 
 import javax.jcr.PropertyType;
 import javax.jcr.ReferentialIntegrityException;
@@ -48,6 +49,7 @@
 import org.apache.jackrabbit.core.virtual.VirtualItemStateProvider;
 import org.apache.jackrabbit.spi.Name;
 import org.apache.jackrabbit.spi.commons.name.NameConstants;
+import org.apache.jackrabbit.JcrConstants;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -631,7 +633,7 @@
                                                 return false;
                                             }
                                             NodeDef def = ntReg.getNodeDef(ns.getDefinitionId());
-                                            return def != null ? def.allowsSameNameSiblings() : false;
+                                            return def != null && def.allowsSameNameSiblings();
                                         }
                                     };
 
@@ -660,9 +662,69 @@
                     shared.deleted(state.getOverlayedState());
                 }
                 for (ItemState state : local.addedStates()) {
+                    if (state.isNode() && hasItemState(state.getId())) {
+                        // node with same id already exists,
+                        // probably caused by mid-air collision of
+                        // concurrent versioning operations (JCR-2272)
+                        if (state.getStatus() != ItemState.STATUS_NEW) {
+                            String msg = state.getId() + " has been created externally";
+                            log.debug(msg);
+                            throw new StaleItemStateException(msg);
+                        }
+
+                        state.connect(getItemState(state.getId()));
+                        // update modification count (will be persisted as well)
+                        state.getOverlayedState().touch();
+                        NodeStateMerger.MergeContext context =
+                                new NodeStateMerger.MergeContext() {
+                                    public boolean isAdded(ItemId id) {
+                                        try {
+                                            ItemState is = local.get(id);
+                                            return is != null
+                                                    && is.getStatus() == ItemState.STATUS_NEW;
+                                        } catch (NoSuchItemStateException e) {
+                                            return false;
+                                        }
+                                    }
+
+                                    public boolean isDeleted(ItemId id) {
+                                        return local.deleted(id);
+                                    }
+
+                                    public boolean isModified(ItemId id) {
+                                        return local.isModified(id);
+                                    }
+
+                                    public boolean allowsSameNameSiblings(NodeId id) {
+                                        NodeState ns;
+                                        try {
+                                            if (local.has(id)) {
+                                                ns = (NodeState) local.get(id);
+                                            } else {
+                                                ns = (NodeState) getItemState(id);
+                                            }
+                                        } catch (ItemStateException e) {
+                                            return false;
+                                        }
+                                        NodeDef def = ntReg.getNodeDef(ns.getDefinitionId());
+                                        return def != null && def.allowsSameNameSiblings();
+                                    }
+                                };
+
+                        if (NodeStateMerger.merge((NodeState) state, context)) {
+                            state.setStatus(ItemState.STATUS_EXISTING_MODIFIED);
+                            shared.modified(state.getOverlayedState());
+                        } else {
+                            String msg = state.getId() + " has been created externally";
+                            log.debug(msg);
+                            throw new StaleItemStateException(msg);
+                        }
+
+                    } else {
-                    state.connect(createInstance(state));
-                    shared.added(state.getOverlayedState());
-                }
+                        state.connect(createInstance(state));
+                        shared.added(state.getOverlayedState());
+                    }
+                }
 
                 // filter out virtual node references for later processing
                 // (see comment above)
Index: jackrabbit-core/src/main/java/org/apache/jackrabbit/core/state/NodeStateMerger.java
===================================================================
--- jackrabbit-core/src/main/java/org/apache/jackrabbit/core/state/NodeStateMerger.java	(revision 792437)
+++ jackrabbit-core/src/main/java/org/apache/jackrabbit/core/state/NodeStateMerger.java	Thu Aug 27 14:50:04 CEST 2009
@@ -69,8 +69,8 @@
                  * - either session moved node
                  */
 
-                // compare current transient state with externally modified
-                // overlayed state and determine what has been changed by whom
+                // compare current state with externally modified overlayed
+                // state and determine what has been changed by whom
 
                 // child node entries order
                 if (!state.getReorderedChildNodeEntries().isEmpty()) {