diff --git a/.github/workflows/pydantic-2.yml b/.github/workflows/pydantic-2.yml
index 948a482..b6aca37 100644
--- a/.github/workflows/pydantic-2.yml
+++ b/.github/workflows/pydantic-2.yml
@@ -27,4 +27,4 @@ jobs:
       - name: Run tests
         shell: bash -el {0}
         run: |
-          cd tests/pydantic-2 && poetry run pytest -Werror
+          cd tests/pydantic-2 && poetry run pytest
diff --git a/.github/workflows/python-3-9.yml b/.github/workflows/python-3-9.yml
index 33dd6d3..784aec7 100644
--- a/.github/workflows/python-3-9.yml
+++ b/.github/workflows/python-3-9.yml
@@ -27,4 +27,4 @@ jobs:
       - name: Run tests
         shell: bash -el {0}
         run: |
-          cd tests/python-3-9 && poetry run pytest -Werror
+          cd tests/python-3-9 && poetry run pytest
diff --git a/substrate/core/coregraph.py b/substrate/core/coregraph.py
index dcbb70b..7eeb652 100644
--- a/substrate/core/coregraph.py
+++ b/substrate/core/coregraph.py
@@ -22,6 +22,10 @@ def __init__(self, id=None, initial_args=None):
     def futures(self) -> List[BaseFuture]:
         raise NotImplementedError
 
+    def add_edge(self, u_node: CoreNode, v_node: CoreNode, **kwargs) -> "CoreGraph":
+        self.DAG.add_edge(u_node, v_node, **kwargs)
+        return self
+
     def add_node(self, node: CoreNode) -> "CoreGraph":
         if self.DAG.has_node(node):
             raise ValueError(f"Node with id {node.id} already exists")
diff --git a/substrate/substrate.py b/substrate/substrate.py
index 8ae0c6f..f0f1fb8 100644
--- a/substrate/substrate.py
+++ b/substrate/substrate.py
@@ -96,7 +96,8 @@ def collect_nodes(node):
 
         graph = Graph()
         for node in all_nodes:
-            graph.add_node(node)
+            if not graph.DAG.has_node(node):
+                graph.add_node(node)
             for depend_node in node._depends:
                 graph.add_edge(depend_node, node)
         graph_serialized = graph.to_dict()