diff --git a/obj2mjcf/cli.py b/obj2mjcf/cli.py
index 77c77ff..559976f 100644
--- a/obj2mjcf/cli.py
+++ b/obj2mjcf/cli.py
@@ -238,7 +238,9 @@ def process_obj(filename: Path, args: Args) -> None:
                 break
         sub_mtls = [smtl]
         mtls = [Material.from_string(smtl)]
-
+    
+    mtls = list(dict.fromkeys(mtls))
+    
     # Delete any MTL files that were created during trimesh processing, if any.
     for file in [
         x for x in work_dir.glob("**/*") if x.is_file() and "material_0" in x.name
diff --git a/obj2mjcf/material.py b/obj2mjcf/material.py
index a61f5a6..dca59b3 100644
--- a/obj2mjcf/material.py
+++ b/obj2mjcf/material.py
@@ -4,7 +4,7 @@
 from obj2mjcf import constants
 
 
-@dataclass
+@dataclass(frozen=True)
 class Material:
     """A convenience class for constructing MuJoCo materials from MTL files."""