diff --git a/doomsday/engine/src/filesys/lumpindex.cpp b/doomsday/engine/src/filesys/lumpindex.cpp
index 011e714123..14e95da0d8 100644
--- a/doomsday/engine/src/filesys/lumpindex.cpp
+++ b/doomsday/engine/src/filesys/lumpindex.cpp
@@ -242,6 +242,8 @@ struct LumpIndex::Instance
 
     void pruneDuplicates()
     {
+        if(!(flags & LIF_NEED_PRUNE_DUPLICATES)) return;
+
         int const numRecords = lumps.size();
         if(numRecords <= 1) return;