diff --git a/src/d_dehacked.cpp b/src/d_dehacked.cpp
index 9e068b5968..dd71fdf756 100644
--- a/src/d_dehacked.cpp
+++ b/src/d_dehacked.cpp
@@ -2985,9 +2985,11 @@ static bool LoadDehSupp ()
 void FinishDehPatch ()
 {
 	unsigned int touchedIndex;
+	unsigned int nameindex = 0;
 
 	for (touchedIndex = 0; touchedIndex < TouchedActors.Size(); ++touchedIndex)
 	{
+		PClassActor *subclass;
 		PClassActor *type = TouchedActors[touchedIndex];
 		AActor *defaults1 = GetDefaultByType (type);
 		if (!(defaults1->flags & MF_SPECIAL))
@@ -2997,9 +2999,16 @@ void FinishDehPatch ()
 
 		// Create a new class that will serve as the actual pickup
 		char typeNameBuilder[32];
-		mysnprintf (typeNameBuilder, countof(typeNameBuilder), "DehackedPickup%d", touchedIndex);
-		PClassActor *subclass = static_cast<PClassActor *>(RUNTIME_CLASS(ADehackedPickup)->
-			CreateDerivedClass(typeNameBuilder, sizeof(ADehackedPickup)));
+		// 
+		do
+		{
+			// Retry until we find a free name. This is unlikely to happen but not impossible.
+			mysnprintf(typeNameBuilder, countof(typeNameBuilder), "DehackedPickup%d", nameindex++);
+			subclass = static_cast<PClassActor *>(RUNTIME_CLASS(ADehackedPickup)->
+				CreateDerivedClass(typeNameBuilder, sizeof(ADehackedPickup)));
+		} 
+		while (subclass == nullptr);
+		
 		AActor *defaults2 = GetDefaultByType (subclass);
 		memcpy ((void *)defaults2, (void *)defaults1, sizeof(AActor));
 
diff --git a/src/dobjtype.cpp b/src/dobjtype.cpp
index c3d0702f37..37ff26fe30 100644
--- a/src/dobjtype.cpp
+++ b/src/dobjtype.cpp
@@ -2861,10 +2861,7 @@ void PClass::InsertIntoHash ()
 	found = TypeTable.FindType(RUNTIME_CLASS(PClass), (intptr_t)Outer, TypeName, &bucket);
 	if (found != NULL)
 	{ // This type has already been inserted
-	  // ... but there is no need whatsoever to make it a fatal error!
-		if (!strictdecorate) Printf (TEXTCOLOR_RED"Tried to register class '%s' more than once.\n", TypeName.GetChars());
-		else I_Error("Tried to register class '%s' more than once.\n", TypeName.GetChars());
-		TypeTable.ReplaceType(this, found, bucket);
+		I_Error("Tried to register class '%s' more than once.\n", TypeName.GetChars());
 	}
 	else
 	{
@@ -3029,15 +3026,23 @@ PClass *PClass::CreateDerivedClass(FName name, unsigned int size)
 	PClass *existclass = static_cast<PClass *>(TypeTable.FindType(RUNTIME_CLASS(PClass), /*FIXME:Outer*/0, name, &bucket));
 
 	// This is a placeholder so fill it in
-	if (existclass != NULL && (existclass->Size == TentativeClass))
+	if (existclass != nullptr)
 	{
-		type = const_cast<PClass*>(existclass);
-		if (!IsDescendantOf(type->ParentClass))
+		if (existclass->Size == TentativeClass)
 		{
-			I_Error("%s must inherit from %s but doesn't.", name.GetChars(), type->ParentClass->TypeName.GetChars());
+			type = const_cast<PClass*>(existclass);
+			if (!IsDescendantOf(type->ParentClass))
+			{
+				I_Error("%s must inherit from %s but doesn't.", name.GetChars(), type->ParentClass->TypeName.GetChars());
+			}
+			DPrintf(DMSG_SPAMMY, "Defining placeholder class %s\n", name.GetChars());
+			notnew = true;
+		}
+		else
+		{
+			// a different class with the same name already exists. Let the calling code deal with this.
+			return nullptr;
 		}
-		DPrintf(DMSG_SPAMMY, "Defining placeholder class %s\n", name.GetChars());
-		notnew = true;
 	}
 	else
 	{
diff --git a/src/thingdef/olddecorations.cpp b/src/thingdef/olddecorations.cpp
index 3ea0c4788a..9afbfc965a 100644
--- a/src/thingdef/olddecorations.cpp
+++ b/src/thingdef/olddecorations.cpp
@@ -140,6 +140,7 @@ DEFINE_CLASS_PROPERTY(respawns, 0, FakeInventory)
 // Reads an old style decoration object
 //
 //==========================================================================
+PClassActor *DecoDerivedClass(const FScriptPosition &sc, PClassActor *parent, FName typeName);
 
 void ParseOldDecoration(FScanner &sc, EDefinitionType def)
 {
@@ -154,7 +155,7 @@ void ParseOldDecoration(FScanner &sc, EDefinitionType def)
 
 	sc.MustGetString();
 	typeName = FName(sc.String);
-	type = static_cast<PClassActor *>(parent->CreateDerivedClass (typeName, parent->Size));
+	type = DecoDerivedClass(FScriptPosition(sc), parent, typeName);
 	ResetBaggage(&bag, parent);
 	bag.Info = type;
 #ifdef _DEBUG
diff --git a/src/thingdef/thingdef.cpp b/src/thingdef/thingdef.cpp
index 32d4f1ce0b..41e1e95422 100644
--- a/src/thingdef/thingdef.cpp
+++ b/src/thingdef/thingdef.cpp
@@ -68,13 +68,44 @@
 
 TDeletingArray<class FxExpression *> ActorDamageFuncs;
 
+
 // EXTERNAL FUNCTION PROTOTYPES --------------------------------------------
 void InitThingdef();
-void ParseDecorate (FScanner &sc);
+void ParseDecorate(FScanner &ctx);
 
 // STATIC FUNCTION PROTOTYPES --------------------------------------------
 PClassActor *QuestItemClasses[31];
 
+EXTERN_CVAR(Bool, strictdecorate);
+
+PClassActor *DecoDerivedClass(const FScriptPosition &sc, PClassActor *parent, FName typeName)
+{
+	PClassActor *type = static_cast<PClassActor *>(parent->CreateDerivedClass(typeName, parent->Size));
+	if (type == nullptr)
+	{
+		FString newname = typeName.GetChars();
+		FString sourcefile = sc.FileName;
+
+		sourcefile.Substitute(":", "@");
+		newname << '@' << sourcefile;
+		if (strictdecorate)
+		{
+			sc.Message(MSG_ERROR, "Tried to define class '%s' more than once.", typeName.GetChars());
+		}
+		else
+		{
+			// Due to backwards compatibility issues this cannot be an unconditional error.
+			sc.Message(MSG_WARNING, "Tried to define class '%s' more than once. Renaming class to '%s'", typeName.GetChars(), newname.GetChars());
+		}
+		type = static_cast<PClassActor *>(parent->CreateDerivedClass(newname, parent->Size));
+		if (type == nullptr)
+		{
+			// This we cannot handle cleanly anymore. Let's just abort and forget about the odd mod out that was this careless.
+			sc.Message(MSG_FATAL, "Tried to define class '%s' more than twice in the same file.", typeName.GetChars());
+		}
+	}
+	return type;
+}
 //==========================================================================
 //
 // Starts a new actor definition
@@ -141,7 +172,7 @@ PClassActor *CreateNewActor(const FScriptPosition &sc, FName typeName, FName par
 	else
 	{
 	create:
-		ti = static_cast<PClassActor *>(parent->CreateDerivedClass (typeName, parent->Size));
+		ti = DecoDerivedClass(sc, parent, typeName);
 	}
 
 	ti->Replacee = ti->Replacement = NULL;
@@ -428,6 +459,16 @@ void LoadActors ()
 	while ((lump = Wads.FindLump ("DECORATE", &lastlump)) != -1)
 	{
 		FScanner sc(lump);
+
+		if (Wads.GetLumpFile(lump) == 0)
+		{
+			// define namespace 'zdoom'
+		}
+		else
+		{
+			// use namespace 'zdoom'
+		}
+
 		ParseDecorate (sc);
 	}
 	FinishThingdef();