Problem: websocket client get duplicated messages

[yihuang]

System info: ethermint main

Steps to reproduce:

crypto-org-chain/cronos#356

1. open two websocket client, and send:

{"id": 2, "method": "eth_subscribe", "params": ["newHeads", {}]}

2. both clients receive two messages for each new block.

Expected behavior: client should receive one message for each block.

Actual behavior: [What actually happened]

Additional info: [Include gist of relevant config, logs, etc.]