diff --git a/SLS_C/src/builtin.c b/SLS_C/src/builtin.c
index e7f84c5..cd6f897 100644
--- a/SLS_C/src/builtin.c
+++ b/SLS_C/src/builtin.c
@@ -515,7 +515,7 @@ Boolean load_builtins(InterpreterState *interpreter_state) {
         break; \
     case STACK_TOKEN_STRING: \
         return FALSE; \
-    }
+    } \
 
 #define NUMERIC_TYPES \
     double af = 0, bf = 0; \
@@ -1445,8 +1445,60 @@ Boolean builtin_drop(InterpreterState *interpreter_state) {
 }
 
 Boolean builtin_dup(InterpreterState *interpreter_state) {
-    (void)interpreter_state;
-    return FALSE;
+    if (interpreter_state->stack == NULL) return FALSE;
+
+    StackItem *item = (StackItem *)malloc(sizeof(StackItem));
+    if (item == NULL) return FALSE;
+    item->type = interpreter_state->stack->type;
+    item->next = interpreter_state->stack;
+
+    switch (interpreter_state->stack->type) {
+    case STACK_IDENTIFIER:
+        item->identifier = interpreter_state->stack->identifier;
+        break;
+    case STACK_I64:
+        item->i64 = interpreter_state->stack->i64;
+        break;
+    case STACK_I32:
+        item->i32 = interpreter_state->stack->i32;
+        break;
+    case STACK_I16:
+        item->i16 = interpreter_state->stack->i16;
+        break;
+    case STACK_I8:
+        item->i8 = interpreter_state->stack->i8;
+        break;
+    case STACK_U64:
+        item->u64 = interpreter_state->stack->u64;
+        break;
+    case STACK_U32:
+        item->u32 = interpreter_state->stack->u32;
+        break;
+    case STACK_U16:
+        item->u16 = interpreter_state->stack->u16;
+        break;
+    case STACK_U8:
+        item->u8 = interpreter_state->stack->u8;
+        break;
+    case STACK_FLOAT:
+        item->f32 = interpreter_state->stack->f32;
+        break;
+    case STACK_DOUBLE:
+        item->f64 = interpreter_state->stack->f64;
+        break;
+    case STACK_CHARACTER:
+        item->character = interpreter_state->stack->character;
+        break;
+    case STACK_BOOLEAN:
+        item->boolean = interpreter_state->stack->boolean;
+        break;
+    case STACK_TOKEN_STRING:
+        item->token_string = copy_token_string(interpreter_state->stack->token_string);
+        break;
+    }
+
+    interpreter_state->stack = item;
+    return TRUE;
 }
 
 Boolean builtin_eval(InterpreterState *interpreter_state) {