Android : What is the link between GCM and a restart of my own server?

Context:

• An Android app (Cordova)


• My own server


• GCM

Some users complained about the issue of duplicated messages received in their Android device.

Indeed, server logs showed duplicated notifications, whose origin is GCM.

The classical issue regarding canonicalId issue would not be an assumption since I already handle it. So I just tried to restart my own server, and the problem merely disappeared...

I wonder why?

What is the link between my own server restart and GCM ?