Firebase Hosting Error: Cannot remove headers after they are sent to the client

Not very sure why I keep getting this error when I tried to create a new static HTML file.

Error [ERR_HTTP_HEADERS_SENT]: Cannot remove headers after they are sent to the client
    at ServerResponse.removeHeader (_http_outgoing.js:595:11)
    at Responder.handleNotModified (/usr/local/lib/node_modules/firebase-tools/node_modules/superstatic/lib/responder.js:138:12)
    at /usr/local/lib/node_modules/firebase-tools/node_modules/superstatic/lib/responder.js:100:19
    at tryCatch (/usr/local/lib/node_modules/firebase-tools/node_modules/rsvp/dist/rsvp.js:525:12)
    at invokeCallback (/usr/local/lib/node_modules/firebase-tools/node_modules/rsvp/dist/rsvp.js:538:13)
    at publish (/usr/local/lib/node_modules/firebase-tools/node_modules/rsvp/dist/rsvp.js:508:7)
    at flush (/usr/local/lib/node_modules/firebase-tools/node_modules/rsvp/dist/rsvp.js:2415:5)
    at processTicksAndRejections (internal/process/task_queues.js:79:11)

In my firebase.json file, I have

{
   "source":"/users/**",
   "destination": "/user.html"
}

and in my public folder, my user.html file:

<!DOCTYPE html>
<html lang="en">

<head>
</head>
<body>
  <p>Welcome to my site!</p>
</body>

I've googled around, but I am still super confused about why this happened. Any direction or help is greatly appreciated.