broke out handleId by itself rather than joined with nodeID, modded necessary functions to give handles not assigned IDs an empty string for backwards compadibility. Also modded handles so that only target-source and source-target connections are allowed, and no repeat connections are allowed.

src/utils/graph.ts

@@ -40,7 +40,19 @@ export const removeElements = (elementsToRemove: Elements, elements: Elements):
   });
 };
 
-const getEdgeId = ({ source, target }: Connection): ElementId => `reactflow__edge-${source}-${target}`;
+const getEdgeId = ({ source, sourceHandle, target, targetHandle }: Connection): ElementId => `reactflow__edge-${source}${sourceHandle}-${target}${targetHandle}`;
+
+const existingConnection = (edge: Edge, elements: Elements) => {
+  for (const element of elements) {
+    if (isEdge(element)) {
+      if (element.source === edge.source && element.sourceHandle === edge.sourceHandle && element.target === edge.target && element.targetHandle === edge.targetHandle) {
+        return true
+      }
+    }
+  }
+  return false
+}
+
 
 export const addEdge = (edgeParams: Edge | Connection, elements: Elements): Elements => {
   if (!edgeParams.source || !edgeParams.target) {
@@ -49,22 +61,27 @@ export const addEdge = (edgeParams: Edge | Connection, elements: Elements): Elem
 
   // make sure that there is node with the target and one with the source id
   [edgeParams.source, edgeParams.target].forEach((id) => {
-    const nodeId = id.includes('__') ? id.split('__')[0] : id;
-    if (!elements.find((e) => isNode(e) && e.id === nodeId)) {
-      throw new Error(`Can't create edge. Node with id=${nodeId} does not exist.`);
+    if (!elements.find((e) => isNode(e) && e.id === id)) {
+      throw new Error(`Can't create edge. Node with id=${id} does not exist.`);
     }
   });
+  // need to check if handles exist
+  // 
+  // 
+  // 
 
+  let edge: Edge
   if (isEdge(edgeParams)) {
-    return elements.concat({ ...edgeParams });
+    edge = {...edgeParams}
+  } else {
+    edge = {
+      ...edgeParams,
+      id: getEdgeId(edgeParams),
+    } as Edge;
   }
-
-  const edge = {
-    ...edgeParams,
-    id: getEdgeId(edgeParams),
-  } as Edge;
-
-  return elements.concat(edge);
+  
+  if (existingConnection(edge, elements)) {return elements}
+  return elements.concat(edge)
 };
 
 export const pointToRendererPoint = (
@@ -112,6 +129,8 @@ export const parseElement = (element: Node | Edge): Node | Edge => {
       ...element,
       source: element.source.toString(),
       target: element.target.toString(),
+      sourceHandle: element.sourceHandle ? element.sourceHandle.toString() : '',
+      targetHandle: element.targetHandle ? element.targetHandle.toString() : '',
       id: element.id.toString(),
       type: element.type || 'default',
     };