diff --git a/common/thorhelper/roxierow.cpp b/common/thorhelper/roxierow.cpp
index 48d297361be..ea4d7706e42 100644
--- a/common/thorhelper/roxierow.cpp
+++ b/common/thorhelper/roxierow.cpp
@@ -488,9 +488,12 @@ class CAllocatorCache : public CSimpleInterfaceOf<IRowAllocatorMetaActIdCache>
             CAllocatorCacheItem *container = _lookup(meta, activityId, flags);
             if (container)
             {
-                if (0 == (roxiemem::RHFunique & flags))
+                if (0 == ((roxiemem::RHFunique|roxiemem::RHFblocked) & flags))
                     return LINK(&container->queryElement());
-                // if in cache but unique, reuse allocatorId
+                // If in cache but unique, reuse allocatorId, but create a unique allocator (and heap)
+                // If blocked the allocator must not be commoned up!  (The underlying heap will be within roxiemem.)
+                // This is very unusual, but can happen if a library is used more than once within the same query
+                // since you will have multiple activity instances with the same activityId.
                 SpinUnblock b(allAllocatorsLock);
                 return callback->createAllocator(this, meta, activityId, container->queryAllocatorId(), flags);
             }