Error: Can't set headers after they are sent in firebase cloud function

I am using Firebase database for my android application. I am using cloud functions feature of it. I have coded functions and trying to test it on local machine.

I am running this command to test it, cloud functions works well on firebase server not on localhost. Below commands sends an error

firebase serve --only functions

> firebase serve --only functions

=== Serving from 'D:\Firebase'...

i  functions: Preparing to emulate HTTPS functions. Support for other event types coming soon.
Warning: You're using Node.js v7.10.0 but Google Cloud Functions only supports v6.11.1.
Server#addProtoService is deprecated. Use addService instead
+  functions: getUsers: http://localhost:5002/xyz/us-central1/getUsers
info: User function triggered, starting execution
info: Execution took 60022 ms, finished with status: 'timeout'
info: Execution took 60022 ms, finished with status: 'crash'
Error: Can't set headers after they are sent.
    at ServerResponse.setHeader (_http_outgoing.js:371:11)
    at ServerResponse.header (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\response.js:730:10)
    at ServerResponse.send (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\response.js:170:12)
    at ServerResponse.json (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\response.js:256:15)
    at ProxyServer.Supervisor._proxy.on (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\@google-cloud\functions-emulator\src\supervisor\supervisor.js:97:12)
    at ProxyServer.emit (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\eventemitter3\index.js:144:27)
    at ClientRequest.proxyError (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\http-proxy\lib\http-proxy\passes\web-incoming.js:156:18)
    at emitOne (events.js:96:13)
    at ClientRequest.emit (events.js:191:7)
    at Socket.socketErrorListener (_http_client.js:358:9)
info: Execution took 60054 ms, finished with status: 'crash'
Error: Can't set headers after they are sent.
    at ServerResponse.setHeader (_http_outgoing.js:371:11)
    at ServerResponse.header (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\response.js:730:10)
    at ServerResponse.send (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\response.js:170:12)
    at ServerResponse.json (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\response.js:256:15)
    at ProxyServer.Supervisor._proxy.on (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\@google-cloud\functions-emulator\src\supervisor\supervisor.js:97:12)
    at ProxyServer.emit (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\eventemitter3\index.js:144:27)
    at ClientRequest.proxyError (C:\Users\williams\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\http-proxy\lib\http-proxy\passes\web-incoming.js:156:18)
    at emitOne (events.js:96:13)
    at ClientRequest.emit (events.js:191:7)
    at Socket.socketCloseListener (_http_client.js:334:9)

Code:

exports.getUsers = functions.https.onRequest((request, response) => {
  var x = [],
    similarityCount;

  db
    .ref("/users/" + request.query.userId)
    .once("value")
    .then(function(snapshot) {
      var jsonObject = snapshot.val();
      var basicProfileJsonObject = jsonObject.basicProfile;
      for (var key in basicProfileJsonObject) {
        if (utils.isNumber(basicProfileJsonObject[key])) {
          x.push(basicProfileJsonObject[key]);
        }
      }
      db.ref("/users/").once("value").then(function(snapshot) {
        var y = [];
        var jsonResponse = [];
        snapshot.forEach(function(item) {
          var user = item.val();
          let userId = user.basicProfile.userId;
          if (userId !== request.query.userId) {
            var basicProfileJsonObject = user.basicProfile;
            for (var key in basicProfileJsonObject) {
              if (utils.isNumber(basicProfileJsonObject[key])) {
                y.push(basicProfileJsonObject[key]);
              }
            }

            if (request.query.algo === "cosine") {
              // compute cosine value
              similarityCount = cosineUtils.cosineSimilarity(x, y);
            } else if (request.query.algo == "euclidean") {
              // compute euclidean distance value
              similarityCount = 1 / (1 + euclidean(x, y));
            } else if (request.query.algo === "pearson-correlation") {
              // compute pearson correlation coefficents
              similarityCount = pcorr.pearsonCorrelation(x, y);
            }
            console.log("------------------------------------");
            console.log(x);
            console.log("------------------------------------");
            console.log("------------------------------------");
            console.log(y);
            console.log("------------------------------------");
            console.log("------------------------------------");
            console.log(similarityCount);
            console.log("------------------------------------");
            jsonResponse.push(item.val());
            y = [];
          }
        });
        response.send(jsonResponse);
      });
    });
});

Can anyone help me what is going wrong ?