Serial.println() duplicated in web editor?

Development Tools Cloud Editor
2

FYI, checked with a serial terminal app and I saw correct output:
tick 1
tick -1
tick 1
.... etc
So looks like it's a but in Web Editor or Windows Create Agent.

I'm using Windows 10 x64, Agent 1.1.69-6d4a183, browser is Chrome Canary 69.0.3444.0