I have created a websocket server, successfully connected it to the Minecraft server, sent the appropriate events to subscribe to PlayerMessage and such, and have been getting double the events through the socket per event in the game itself. 

I have tested this same code in 1.8, and it works as intended - one event per message. 

I have included a small test case in the attachments. It requires Node 8+ and npm/yarn. Run yarn or npm install, and then node index.js. It will open a websocket on port 1337. Run /wsserver localhost:1337 in 1.9.0.3 and then type a message. For every single message sent, it will send back a message as well. Notice in 1.9.0.3 it sends two for every one, whereas in 1.8 it returns one for one.