diff --git a/pkg/client/client.go b/pkg/client/client.go
index 86ed0a99e..73f75db60 100644
--- a/pkg/client/client.go
+++ b/pkg/client/client.go
@@ -1194,13 +1194,6 @@ func (c *KpmClient) downloadDeps(deps pkg.Dependencies, lockDeps pkg.Dependencie
 
 	depGraph := graph.New(graph.StringHash, graph.Directed())
 
-	for _, d := range newDeps.Deps {
-		err := depGraph.AddVertex(fmt.Sprintf("%s@%s", d.Name, d.Version))
-		if err != nil {
-			return nil, nil, err
-		}
-	}
-
 	// Recursively download the dependencies of the new dependencies.
 	for _, d := range newDeps.Deps {
 		// Load kcl.mod file of the new downloaded dependencies.
@@ -1223,12 +1216,19 @@ func (c *KpmClient) downloadDeps(deps pkg.Dependencies, lockDeps pkg.Dependencie
 		}
 
 		source := fmt.Sprintf("%s@%s", d.Name, d.Version)
+		err = depGraph.AddVertex(source)
+		if err != nil {
+			if err != graph.ErrVertexAlreadyExists {
+				return nil, nil, err
+			}
+		}
+
 		sourcesOfNestedDepGraph, err := FindSource(nestedDepGraph)
 		if err != nil {
 			return nil, nil, err
 		}
 
-		depGraph, err = graph.Union(depGraph, nestedDepGraph)
+		depGraph, err = Union(depGraph, nestedDepGraph)
 		if err != nil {
 			return nil, nil, err
 		}
@@ -1236,7 +1236,7 @@ func (c *KpmClient) downloadDeps(deps pkg.Dependencies, lockDeps pkg.Dependencie
 		// make an edge between the source of all nested dep graph and main dep graph 
 		for _, sourceOfNestedDepGraph := range sourcesOfNestedDepGraph {
 			err = depGraph.AddEdge(source, sourceOfNestedDepGraph)
-			if err != nil {
+			if err != nil && err != graph.ErrEdgeAlreadyExists {
 				return nil, nil, err
 			}
 		}
@@ -1341,3 +1341,61 @@ func FindSource[K comparable, T any](g graph.Graph[K, T]) ([]K, error) {
 	}
 	return sources, nil
 }
+
+// Union combines two given graphs into a new graph. The vertex hashes in both
+// graphs are expected to be unique. The two input graphs will remain unchanged.
+//
+// Both graphs should be either directed or undirected. All traits for the new
+// graph will be derived from g.
+// 
+// If the same vertex happens to be in both g and h, then an error will not be 
+// thrown as happens in original Union function and successful operation takes place.
+func Union[K comparable, T any](g, h graph.Graph[K, T]) (graph.Graph[K, T], error) {
+	union, err := g.Clone()
+	if err != nil {
+		return union, fmt.Errorf("failed to clone g: %w", err)
+	}
+
+	adjacencyMap, err := h.AdjacencyMap()
+	if err != nil {
+		return union, fmt.Errorf("failed to get adjacency map: %w", err)
+	}
+
+	addedEdges := make(map[K]map[K]struct{})
+
+	for currentHash := range adjacencyMap {
+		vertex, err := h.Vertex(currentHash)
+		if err != nil {
+			return union, fmt.Errorf("failed to get vertex %v: %w", currentHash, err)
+		}
+
+		err = union.AddVertex(vertex)
+		if err != nil && err != graph.ErrVertexAlreadyExists {
+			return union, fmt.Errorf("failed to add vertex %v: %w", currentHash, err)
+		}
+	}
+
+	for _, adjacencies := range adjacencyMap {
+		for _, edge := range adjacencies {
+			if _, sourceOK := addedEdges[edge.Source]; sourceOK {
+				if _, targetOK := addedEdges[edge.Source][edge.Target]; targetOK {
+					// If the edge addedEdges[source][target] exists, the edge
+					// has already been created and thus can be skipped here.
+					continue
+				}
+			}
+
+			err = union.AddEdge(edge.Source, edge.Target)
+			if err != nil {
+				return union, fmt.Errorf("failed to add edge (%v, %v): %w", edge.Source, edge.Target, err)
+			}
+
+			if _, ok := addedEdges[edge.Source]; !ok {
+				addedEdges[edge.Source] = make(map[K]struct{})
+			}
+			addedEdges[edge.Source][edge.Target] = struct{}{}
+		}
+	}
+
+	return union, nil
+}