diff --git a/src/tdastro/base_models.py b/src/tdastro/base_models.py
index 886fa9e..959acf6 100644
--- a/src/tdastro/base_models.py
+++ b/src/tdastro/base_models.py
@@ -472,7 +472,7 @@ def _sample_helper(self, graph_state, seen_nodes, rng_info=None):
             An object mapping graph parameters to their values. This object is modified
             in place as it is sampled.
         seen_nodes : `dict`
-            A dictionary mapping nodes seen during this sampling run to their ID.
+            A dictionary mapping nodes strings seen during this sampling run to their object.
             Used to avoid sampling nodes multiple times and to validity check the graph.
         rng_info : numpy.random._generator.Generator, optional
             A given numpy random number generator to use for this computation. If not
@@ -482,9 +482,12 @@ def _sample_helper(self, graph_state, seen_nodes, rng_info=None):
         ------
         Raise a ``KeyError`` if the sampling encounters an error with the order of dependencies.
         """
-        if self in seen_nodes:
+        node_str = str(self)
+        if node_str in seen_nodes:
+            if seen_nodes[node_str] != self:
+                raise ValueError(f"Duplicate node label {node_str}.")
             return  # Nothing to do
-        seen_nodes[self] = self.node_pos
+        seen_nodes[node_str] = self
 
         # Run through each parameter and sample it based on the given recipe.
         # As of Python 3.7 dictionaries are guaranteed to preserve insertion ordering,
diff --git a/src/tdastro/sources/physical_model.py b/src/tdastro/sources/physical_model.py
index 8b4ee87..3cd43a8 100644
--- a/src/tdastro/sources/physical_model.py
+++ b/src/tdastro/sources/physical_model.py
@@ -144,8 +144,8 @@ def evaluate(self, times, wavelengths, graph_state=None, given_args=None, rng_in
             A length T x N matrix of SED values (in nJy).
         """
         # Make sure times and wavelengths are numpy arrays.
-        times = np.array(times)
-        wavelengths = np.array(wavelengths)
+        times = np.asarray(times)
+        wavelengths = np.asarray(wavelengths)
 
         # Check if we need to sample the graph.
         if graph_state is None:
diff --git a/src/tdastro/sources/sncomso_models.py b/src/tdastro/sources/sncomso_models.py
index 596a815..54dcfc3 100644
--- a/src/tdastro/sources/sncomso_models.py
+++ b/src/tdastro/sources/sncomso_models.py
@@ -36,6 +36,9 @@ class SncosmoWrapperModel(PhysicalModel):
         Any additional keyword arguments.
     """
 
+    # A class variable for the units so we are not computing them each time.
+    _FLAM_UNIT = u.erg / u.second / u.cm**2 / u.AA
+
     def __init__(self, source_name, t0=0.0, node_label=None, **kwargs):
         super().__init__(node_label=node_label, **kwargs)
         self.source_name = source_name
@@ -160,7 +163,7 @@ def compute_flux(self, times, wavelengths, graph_state=None, **kwargs):
             flux_flam,
             wavelengths,
             wave_unit=u.AA,
-            flam_unit=u.erg / u.second / u.cm**2 / u.AA,
+            flam_unit=self._FLAM_UNIT,
             fnu_unit=u.nJy,
         )
 
diff --git a/tests/tdastro/test_base_models.py b/tests/tdastro/test_base_models.py
index 64c5d82..d0766ef 100644
--- a/tests/tdastro/test_base_models.py
+++ b/tests/tdastro/test_base_models.py
@@ -153,6 +153,26 @@ def test_parameterized_node():
     assert model4.get_param(state, "value_sum") != model4.get_param(new_state, "value_sum")
 
 
+def test_parameterized_node_label_collision():
+    """Test that throw an error when two nodes use the same label."""
+    node_a = SingleVariableNode("A", 10.0, node_label="A")
+    node_b = SingleVariableNode("B", 20.0, node_label="B")
+    pair1 = PairModel(value1=node_a.A, value2=node_b.B, node_label="pair1")
+    pair2 = PairModel(value1=pair1.value_sum, value2=node_b.B, node_label="pair2")
+
+    # No collision even though node_b is referenced twice.
+    state = pair2.sample_parameters()
+    assert state["pair1"]["value_sum"] == 30.0
+    assert state["pair2"]["value_sum"] == 50.0
+
+    # We run into a problem if we reuse a label. Label "A" collides multiple
+    # levels above.
+    node_c = SingleVariableNode("C", 5.0, node_label="C")
+    pair3 = PairModel(value1=pair2.value_sum, value2=node_c.C, node_label="A")
+    with pytest.raises(ValueError):
+        _ = pair3.sample_parameters()
+
+
 def test_parameterized_node_modify():
     """Test that we can modify the parameters in a node."""
     model = PairModel(value1=0.5, value2=0.5)