diff --git a/haystack/pipelines/base.py b/haystack/pipelines/base.py
index 538d207500..14677fd20b 100644
--- a/haystack/pipelines/base.py
+++ b/haystack/pipelines/base.py
@@ -610,7 +610,6 @@ def run(  # type: ignore
                                     updated_input["meta"] = meta
                             else:
                                 existing_input["inputs"].append(node_output)
-                                # TODO This doesn't have an effect until we also pass on keys that only occur once
                                 additional_input = self._combine_node_outputs(existing_input, node_output)
                                 updated_input = {**additional_input, **existing_input}
                             queue[n] = updated_input
@@ -630,13 +629,24 @@ def _combine_node_outputs(self, existing_input: Dict[str, Any], node_output: Dic
         :param node_output: The output of the second node.
         """
         additional_input = {}
-        # Pass keys that appear in both inputs that have the same values
+        # TODO Should we support overwriting keys that exist in both? --> first node's value is kept
+        # Add shared items from existing_input and node_output that have matching values
         shared_items = {
             k: existing_input[k] for k in existing_input if k in node_output and existing_input[k] == node_output[k]
         }
         for key in shared_items:
-            if key != "inputs" or key != "params" or key != "_debug":
+            if key not in ["inputs", "params", "_debug"]:
                 additional_input[key] = shared_items[key]
+        unique_existing_input = {k: v for k, v in existing_input.items() if k not in shared_items}
+        # Add unique keys from existing_input
+        for key in unique_existing_input:
+            if key not in ["inputs", "params", "_debug"]:
+                additional_input[key] = unique_existing_input[key]
+        # Add unique keys from node_output
+        unique_node_output = {k: v for k, v in node_output.items() if k not in shared_items}
+        for key in unique_node_output:
+            if key not in ["inputs", "params", "_debug"]:
+                additional_input[key] = unique_node_output[key]
         return additional_input
 
     async def _arun(  # noqa: C901,PLR0912 type: ignore
diff --git a/test/pipelines/test_pipeline.py b/test/pipelines/test_pipeline.py
index a454891534..d8239bfefe 100644
--- a/test/pipelines/test_pipeline.py
+++ b/test/pipelines/test_pipeline.py
@@ -2143,15 +2143,18 @@ def test_pipeline_execution_using_join_preserves_previous_keys_three_streams():
     document_store_3.write_documents(dicts_3)
 
     # Create Shaper to insert "invocation_context" and "test_key" into the node_output
-    shaper = Shaper(func="rename", inputs={"value": "query"}, outputs=["test_key"])
+    shaper1 = Shaper(func="rename", inputs={"value": "query"}, outputs=["test_key1"])
+    shaper2 = Shaper(func="rename", inputs={"value": "query"}, outputs=["test_key2"])
 
     pipeline = Pipeline()
-    pipeline.add_node(component=shaper, name="Shaper", inputs=["Query"])
-    pipeline.add_node(component=retriever_1, name="Retriever1", inputs=["Shaper"])
-    pipeline.add_node(component=retriever_2, name="Retriever2", inputs=["Shaper"])
-    pipeline.add_node(component=retriever_3, name="Retriever3", inputs=["Shaper"])
+    pipeline.add_node(component=shaper1, name="Shaper1", inputs=["Query"])
+    pipeline.add_node(component=shaper2, name="Shaper2", inputs=["Query"])
+    pipeline.add_node(component=retriever_3, name="Retriever3", inputs=["Shaper2"])
+    pipeline.add_node(component=retriever_1, name="Retriever1", inputs=["Shaper1"])
+    pipeline.add_node(component=retriever_2, name="Retriever2", inputs=["Shaper1"])
+
     pipeline.add_node(
-        component=JoinDocuments(join_mode="concatenate"), name="Join", inputs=["Retriever1", "Retriever2", "Retriever3"]
+        component=JoinDocuments(join_mode="concatenate"), name="Join", inputs=["Retriever3", "Retriever1", "Retriever2"]
     )
     res = pipeline.run(query="Alpha Beta Gamma Delta")
     assert set(res.keys()) == {
@@ -2159,13 +2162,15 @@ def test_pipeline_execution_using_join_preserves_previous_keys_three_streams():
         "labels",
         "root_node",
         "params",
-        "test_key",
+        "test_key1",
+        "test_key2",
         "invocation_context",
         "query",
         "node_id",
     }
-    assert res["test_key"] == "Alpha Beta Gamma Delta"
-    assert res["invocation_context"] == {"query": "Alpha Beta Gamma Delta", "test_key": "Alpha Beta Gamma Delta"}
+    assert res["test_key1"] == "Alpha Beta Gamma Delta"
+    assert res["test_key2"] == "Alpha Beta Gamma Delta"
+    assert res["invocation_context"] == {"query": "Alpha Beta Gamma Delta", "test_key1": "Alpha Beta Gamma Delta"}
     assert len(res["documents"]) == 3