diff --git a/src/common/filesystem/filesystem.cpp b/src/common/filesystem/filesystem.cpp
index 61c947c7a..ff991d8b8 100644
--- a/src/common/filesystem/filesystem.cpp
+++ b/src/common/filesystem/filesystem.cpp
@@ -208,7 +208,7 @@ void FileSystem::InitSingleFile(const char* filename, bool quiet)
 	InitMultipleFiles(filenames, true);
 }
 
-void FileSystem::InitMultipleFiles (TArray<FString> &filenames, bool quiet, LumpFilterInfo* filter)
+void FileSystem::InitMultipleFiles (TArray<FString> &filenames, bool quiet, LumpFilterInfo* filter, bool allowduplicates)
 {
 	int numfiles;
 
@@ -217,14 +217,17 @@ void FileSystem::InitMultipleFiles (TArray<FString> &filenames, bool quiet, Lump
 	numfiles = 0;
 
 	// first, check for duplicates
-	if (!Args->CheckParm("-allowduplicates"))
+	if (allowduplicates)
 	{
 		for (unsigned i=0;i<filenames.Size(); i++)
 		{
 			for (unsigned j=i+1;j<filenames.Size(); j++)
 			{
 				if (strcmp(filenames[i], filenames[j]) == 0)
+				{
 					filenames.Delete(j);
+					j--;
+				}
 			}
 		}
 	}
@@ -1697,4 +1700,4 @@ CCMD(fs_dir)
 		bool hidden = fileSystem.FindFile(fn1) != i;
 		Printf(PRINT_NONOTIFY, "%s%-64s %-15s (%5d) %10d %s %s\n", hidden ? TEXTCOLOR_RED : TEXTCOLOR_UNTRANSLATED, fn1, fns, fnid, length, container, hidden ? "(h)" : "");
 	}
-}
\ No newline at end of file
+}
diff --git a/src/common/filesystem/filesystem.h b/src/common/filesystem/filesystem.h
index 5924e86c1..4052a7f4c 100644
--- a/src/common/filesystem/filesystem.h
+++ b/src/common/filesystem/filesystem.h
@@ -68,7 +68,7 @@ public:
 	void SetMaxIwadNum(int x) { MaxIwadIndex = x; }
 
 	void InitSingleFile(const char *filename, bool quiet = false);
-	void InitMultipleFiles (TArray<FString> &filenames, bool quiet = false, LumpFilterInfo* filter = nullptr);
+	void InitMultipleFiles (TArray<FString> &filenames, bool quiet = false, LumpFilterInfo* filter = nullptr, bool allowduplicates = false);
 	void AddFile (const char *filename, FileReader *wadinfo, bool quiet, LumpFilterInfo* filter);
 	int CheckIfResourceFileLoaded (const char *name) noexcept;
 	void AddAdditionalFile(const char* filename, FileReader* wadinfo = NULL) {}
diff --git a/src/d_main.cpp b/src/d_main.cpp
index c18715c00..27f2fa865 100644
--- a/src/d_main.cpp
+++ b/src/d_main.cpp
@@ -3205,7 +3205,8 @@ static int D_DoomMain_Internal (void)
 			FindStrifeTeaserVoices(fileSystem);
 		};
 
-		fileSystem.InitMultipleFiles (allwads, false, &lfi);
+		bool allowduplicates = Args->CheckParm("-allowduplicates");
+		fileSystem.InitMultipleFiles (allwads, false, &lfi, allowduplicates);
 		allwads.Clear();
 		allwads.ShrinkToFit();
 		SetMapxxFlag();