diff --git a/nix-js/Cargo.toml b/nix-js/Cargo.toml
index b4a63b3..6d560c0 100644
--- a/nix-js/Cargo.toml
+++ b/nix-js/Cargo.toml
@@ -11,14 +11,17 @@ mimalloc = "0.1"
 anyhow = "1.0"
 rustyline = "14.0"
 
-regex = "1.11"
-hashbrown = "0.16"
 derive_more = { version = "2", features = ["full"] }
-thiserror = "2"
-string-interner = "0.19"
-itertools = "0.14"
 pin-project = "1"
 
+hashbrown = "0.16"
+string-interner = "0.19"
+
+thiserror = "2"
+itertools = "0.14"
+
+regex = "1.11"
+
 deno_core = "0.376"
 deno_error = "0.7"
 
diff --git a/nix-js/src/ir/utils.rs b/nix-js/src/ir/utils.rs
index 8624f96..1960339 100644
--- a/nix-js/src/ir/utils.rs
+++ b/nix-js/src/ir/utils.rs
@@ -242,7 +242,13 @@ where
             ast::Entry::Inherit(inherit) => {
                 for attr in inherit.attrs() {
                     if let ast::Attr::Ident(ident) = attr {
-                        binding_syms.insert(ctx.new_sym(ident.to_string()));
+                        let sym = ctx.new_sym(ident.to_string());
+                        if !binding_syms.insert(sym) {
+                            return Err(Error::downgrade_error(format!(
+                                "attribute '{}' already defined",
+                                format_symbol(ctx.get_sym(sym))
+                            )));
+                        }
                     }
                 }
             }
@@ -251,7 +257,13 @@ where
                 if let Some(first_attr) = attrpath.attrs().next()
                     && let ast::Attr::Ident(ident) = first_attr
                 {
-                    binding_syms.insert(ctx.new_sym(ident.to_string()));
+                    let sym = ctx.new_sym(ident.to_string());
+                    if !binding_syms.insert(sym) {
+                        return Err(Error::downgrade_error(format!(
+                            "attribute '{}' already defined",
+                            format_symbol(ctx.get_sym(sym))
+                        )));
+                    }
                 }
             }
         }