diff --git a/mesa_frames/concrete/polars/agentset.py b/mesa_frames/concrete/polars/agentset.py
index 4bec1ea5..85b2d498 100644
--- a/mesa_frames/concrete/polars/agentset.py
+++ b/mesa_frames/concrete/polars/agentset.py
@@ -143,7 +143,13 @@ def add(
         if isinstance(obj._mask, pl.Series):
             original_active_indices = obj._agents.filter(obj._mask)["unique_id"]
 
-        obj._agents = pl.concat([obj._agents, new_agents], how="diagonal_relaxed")
+        combined_agents = pl.concat([obj._agents, new_agents], how="diagonal_relaxed")
+        if combined_agents["unique_id"].is_duplicated().any():
+            raise ValueError(
+                "Some ids are duplicated in the AgentSet that are trying to be added together."
+            )
+
+        obj._agents = combined_agents
 
         if isinstance(obj._mask, pl.Series):
             obj._update_mask(original_active_indices, new_agents["unique_id"])
diff --git a/tests/polars/test_agentset_polars.py b/tests/polars/test_agentset_polars.py
index 9c311727..180a5b9d 100644
--- a/tests/polars/test_agentset_polars.py
+++ b/tests/polars/test_agentset_polars.py
@@ -32,6 +32,17 @@ def fix1_AgentSetPolars() -> ExampleAgentSetPolars:
     return agents
 
 
+@pytest.fixture
+def fix4_AgentSetPolars() -> ExampleAgentSetPolars:
+    model = ModelDF()
+    agents = ExampleAgentSetPolars(model)
+    agents.add({"unique_id": [0, 1, 2, 3]})
+    agents["wealth"] = agents.starting_wealth
+    agents["age"] = [10, 20, 30, 40]
+    model.agents.add(agents)
+    return agents
+
+
 @pytest.fixture
 def fix2_AgentSetPolars() -> ExampleAgentSetPolars:
     model = ModelDF()
@@ -73,9 +84,17 @@ def test_add(
         self,
         fix1_AgentSetPolars: ExampleAgentSetPolars,
         fix2_AgentSetPolars: ExampleAgentSetPolars,
+        fix4_AgentSetPolars: ExampleAgentSetPolars,
     ):
         agents = fix1_AgentSetPolars
         agents2 = fix2_AgentSetPolars
+        agents4 = fix4_AgentSetPolars
+
+        with pytest.raises(
+            ValueError,
+            match="Some ids are duplicated in the AgentSet that are trying to be added together.",
+        ):
+            result = agents.add(agents4.agents, inplace=False)
 
         # Test with a DataFrame
         result = agents.add(agents2.agents, inplace=False)