diff --git a/src/lang/connect/connect.ts b/src/lang/connect/connect.ts
index 63a93b51..350f1e62 100644
--- a/src/lang/connect/connect.ts
+++ b/src/lang/connect/connect.ts
@@ -1,4 +1,5 @@
 import { checkPortSigns } from "../check/checkPortSigns"
+import { edgeEqual } from "../edge/edgeEqual"
 import { Net } from "../net"
 import { findPortEntry } from "../net/findPortEntry"
 import { findPortRecordOrFail } from "../net/findPortRecordOrFail"
@@ -48,7 +49,11 @@ export function connect(net: Net, first: Port, second: Port): void {
   const secondPortRecord = findPortRecordOrFail(net, second.node)
   secondPortRecord[second.name].connection = { port: first }
 
-  if (first.isPrincipal && second.isPrincipal) {
+  if (
+    first.isPrincipal &&
+    second.isPrincipal &&
+    !net.activeEdges.find((edge) => edgeEqual(edge, { first, second }))
+  ) {
     net.activeEdges.push(edge)
   }
 }
diff --git a/src/lang/edge/edgeEqual.ts b/src/lang/edge/edgeEqual.ts
new file mode 100644
index 00000000..e0f0bca0
--- /dev/null
+++ b/src/lang/edge/edgeEqual.ts
@@ -0,0 +1,9 @@
+import { portEqual } from "../port/portEqual"
+import { Edge } from "./Edge"
+
+export function edgeEqual(x: Edge, y: Edge): boolean {
+  return (
+    (portEqual(x.first, y.first) && portEqual(x.second, y.second)) ||
+    (portEqual(x.first, y.second) && portEqual(x.second, y.first))
+  )
+}
diff --git a/src/lang/node/nodeEqual.ts b/src/lang/node/nodeEqual.ts
new file mode 100644
index 00000000..80db4354
--- /dev/null
+++ b/src/lang/node/nodeEqual.ts
@@ -0,0 +1,5 @@
+import { Node } from "./Node"
+
+export function nodeEqual(x: Node, y: Node): boolean {
+  return x.name === y.name && x.id === y.id
+}
diff --git a/src/lang/port/portEqual.ts b/src/lang/port/portEqual.ts
new file mode 100644
index 00000000..cd832231
--- /dev/null
+++ b/src/lang/port/portEqual.ts
@@ -0,0 +1,6 @@
+import { nodeEqual } from "../node/nodeEqual"
+import { Port } from "./Port"
+
+export function portEqual(x: Port, y: Port): boolean {
+  return nodeEqual(x.node, y.node) && x.name === y.name
+}
diff --git a/tests/statement/define-without-claim.error.i.err b/tests/statement/define-without-claim.error.i.err
index b414397e..d6cd0c0e 100644
--- a/tests/statement/define-without-claim.error.i.err
+++ b/tests/statement/define-without-claim.error.i.err
@@ -1,4 +1,4 @@
-[lookupDefinitionOrFail] I meet undefined name.
+[findDefinitionOrFail] I meet undefined name.
 
   name: swapswap
 
diff --git a/tests/statement/rule-before-node.error.i.err b/tests/statement/rule-before-node.error.i.err
index 7ea6f90e..fbac249a 100644
--- a/tests/statement/rule-before-node.error.i.err
+++ b/tests/statement/rule-before-node.error.i.err
@@ -1,4 +1,4 @@
-[lookupDefinitionOrFail] I meet undefined name.
+[findDefinitionOrFail] I meet undefined name.
 
   name: add