JWT Verify Causing ERR_HTTP_HEADERS_SENT

We are developing a ReST API using Restify (Node.js). As part of the authentication mechanism, we developed a middleware whose purpose is to screen incoming tokens and verify them. However, looking at the logs, multiple errors related to ERR_HTTP_HEADERS_SENT are popping up.

Here is the code for the middleware:

var client = jwksClient({
    strictSsl: false, 
    jwksUri: process.env.JWKS_URI
})

// Route middleware to verify a token
server.use(
    async (req, res, next) => {

        let path = req.route.path.replace(/\?.*$/,'')

        let url = req.url

        let whitelist = [
            // list of endpoints that are left unprotected
        ]

        if(whitelist.indexOf(path) > -1 || whitelist.indexOf(url) > -1){
            return next()
        } 
        else {
            const r = req
            let token = req.headers['x-access-token'] || req.headers['authorization']
            
            // Decode token
            token = token.slice(7,token.length).trimLeft()
            
            token = token.trim()

            if (token == undefined || token.length <= 0) {
               res.send(401)
               return
            }

            try{
                // Verify the token
                jwt.verify(
                    token, 
                    getKey, 
                    {
                        algorithm: process.env.JWT_ALGORITHM
                    }, 
                    (err, decoded) => {
                        if (err) {
                            res.end(401)
                            return
                        } 
                        else{
                            next()
                        }
                })    
            }
            catch(e){
                res.send(401)
                return
            }

            let email = jwtDecode(token)['email']
    
            // Code here to query database and check whether email is registered
            
            // If no result has been returned
            if (email != null && email != undefined && email.length > 0) {
                return next()
            }
            else {
                res.send(401)
                return
            }
        }   
})

We're using jsonwebtoken as the library to do the verification.

What happen is that your using the callback, and the rest of the function keep going on, so untill the callback happens your already sending the headers

            try{
                // Verify the token
                jwt.verify(
                    token, 
                    getKey, 
                    {
                        algorithm: process.env.JWT_ALGORITHM
                    })
                 return next()
            }
            catch(e){
                res.send(401)
                return
            }