Express Can't set headers after they are sent - json

Question

I've tried to find similar questions with this problem and while there are lots with the same error I don't think I'm getting the error for the same reason.
I have a route, POST /api/servers which creates a server listing in the database. I don't know if it is wrong to include so much logic in an express route but it seems to be working up until this error I keep getting.
Basically, if creating a server that already exists (address + port combo) then the route sends back an error with 'SERVER ALREADY EXISTS' as the message in JSON, but when ever this happens if I try to create a server that already exists in postman I do get the JSON back as expected but my server crashes with 'Cant set headers after they are sent'
router.post('/servers', passport.authenticate('jwt', { session: false }), function(req, res) {
//This is very lazy validation, to do in future version, create error object
// which accumulates the problems with the form and can return all problems at
// once to user
if (!req.body.name) {
return res.json({success: false, message: 'NAME REQUIRED'})
}
if (!req.body.address) {
return res.json({success: false, message: 'ADDRESS REQUIRED'})
}
if (!req.body.image) {
return res.json({success: false, message: 'IMAGE REQUIRED'})
}
if (!req.body.description) {
return res.json({success: false, message: 'DESCRIPTION REQUIRED'})
}
var newServer = new Server();
newServer.name = req.body.name;
newServer.address = req.body.address;
//if a custom port is specified, uses it otherwise defaults to 25565
if (req.body.port) {
newServer.port = req.body.port;
} else {
newServer.port = 25565;
}
//Check if server with address and port not already exists
Server.findOne({ address: newServer.address, port: newServer.port }, function(err, server) {
if (err) {
return res.json({success: false, message: 'Internal Error'});
} else if (server) {
return res.json({success: false, message: 'SERVER ALREADY EXISTS'});
}
});
newServer.image = req.body.image;
newServer.description = req.body.description;
newServer.owner = req.user._id; // Gives the current user id
if (req.body.votifier_enabled) {
//If votifier is enabled on this server
newServer.votifier.enabled = true;
if (!req.body.votifier_address) {
return res.json({success: false, message: 'VOTIFIER ADDRESS REQUIRED'})
}
//if custom votifier port specified, it is used otherwise defaults to 8192
if (req.body.votifier_port) {
newServer.votifier.port = req.body.votifier_port;
} else {
newServer.votifier.port = 8192;
}
if (!req.body.votifier_pubkey) {
return res.json({success: false, message: 'VOTIFIER PUBLIC KEY REQUIRED'})
}
newServer.votifier.address = req.body.votifier_address;
newServer.votifier.pubkey = req.body.votifier_pubkey;
//Check if votifier server with address and port not already exists
Server.findOne({ votifier: { address: newServer.votifier.address, port: newServer.votifier.port } }, function(err, server) {
if (err) {
return res.json({success: false, message: 'Internal Error'});
} else if (server) {
return res.json({success: false, message: 'VOTIFIER ALREADY EXISTS'});
}
});
};
if(!req.body.hidden) {
newServer.hidden = false;
} else {
newServer.hidden = req.body.hidden;
};
console.log('passed all verification');
//Lastly: Verify that the server really does belong to this user,
//checks that string config.VERIFY_OWNERSHIP_STRING is in servers motd
mcping(req.body.address, req.body.port, function(err, stats) {
if (err) {
//Problem connecting to server
return res.json({success: false, message: 'SERVER CONNECTION FAILED: Is your server online?'})
} else {
console.log('pinged server successfully')
var status = stats;
if (status.description.text.includes(config.VERIFY_OWNERSHIP_STRING)){
newServer.status.online = true;
newServer.status.motd = status.description.text;
newServer.status.currPlayers = status.players.online;
newServer.status.maxPlayers = status.players.max;
//Verification successful, server does belong to this user
//Validation complete and newServer object created! Now to save it to database
newServer.save(function(err, server) {
console.log('attempted to save the server');
if (err) {
//Theoretically - this should not happen unless theres an internal error
console.log(err.errors);
return res.json({success: false, message: err});
} else if (server) {
console.log('save successful')
return res.json({success: true, _id: server._id});
}
});
} else {
return res.json({success: false, message: "SERVER VALIDATION FAILED: Is '" + config.VERIFY_OWNERSHIP_STRING + "' in your motd?"});
}
}
}, 3000);
})
This is the route that is causing the problem, I think the problem is because even though the server already exists so the JSON message is being sent back to say SERVER ALREADY EXISTS all the console.logs below still are called, including in the mongoose save function.
I thought that res.json is supposed to end the request response cycle? So I was under the impression it's okay to have so many res.jsons called but now I'm thinking this is the problem
Thanks for any help and sorry for the confusing code!