diff --git a/core/serial_node_ble.js b/core/serial_node_ble.js
index <HASH>..<HASH> 100644
--- a/core/serial_node_ble.js
+++ b/core/serial_node_ble.js
@@ -164,7 +164,10 @@
         receiveCallback(new Uint8Array(data).buffer)
       );
 
-      return rxCharacteristic.startNotifications().then(() =>
+      return rxCharacteristic.isNotifying().then((n) => {
+        if (n)
+          console.error("Another process is connected to this device, problems may occur.");
+      }).then(() => rxCharacteristic.startNotifications()).then(() =>
         openCallback(true)
       );
     }).catch((e) => {
Throw a warning message if WebIDE or another process has the device open.

Currently all received data gets incorrectly duplicated when multiple
processes access the same characteristic.