Express can't set headers after they are sent to the client - express

Question

I have the following code:
router.post('/:email/addWorkflow', async function (req, res, next) {
const params = req.params;
const workflow = req.body;
const email = params.email;
User.findOne({ email: email }, function (err, user) {
if (err) {
res.status(500).send({
error: 'Error while querying database'
});
} else if (user) {
const workflows = user.workflows;
workflows.forEach(wf => {
if (wf) {
if (wf.workflowId === workflow.workflowId) {
res.status(409).send({
error: 'Workflow with that id already exists'
});
}
}
});
workflows.push(workflow);
User.updateOne({ email: email }, { $set: { workflows: workflows } }, { upsert: false }, function (err) {
if (err) {
res.status(500).send({
message: 'Error while updating database'
});
} else {
res.status(200).send({
message: 'Wf added successfully'
});
}
});
} else {
res.status(404).send({
message: 'No such user'
});
}
});
});
After I make a post with an already existing workflowId, I get the following error:
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
at ServerResponse.setHeader (_http_outgoing.js:485:11)
..........
at /home/petar/Documents/jsProjects/p/backend/routes/users.js:50:29
at CoreDocumentArray.forEach (<anonymous>)
at /home/petar/Documents/jsProjects/p/backend/routes/users.js:47:17
at /home/petar/Documents/jsProjects/p/backend/node_modules/mongoose/lib/model.js:4915:16
at /home/petar/Documents/jsProjects/p/backend/node_modules/mongoose/lib/model.js:4915:16
at /home/petar/Documents/jsProjects/linear-mixed-models/backend/node_modules/mongoose/lib/query.js:4380:11
[... lines matching original stack trace ...]
at processTicksAndRejections (internal/process/task_queues.js:76:11) {
code: 'ERR_HTTP_HEADERS_SENT'
Any ideas? I looked at other posts for the same error. I understand that it happens if I try to send response 2 time: res.send({...}) and res.send({...}). However, this does not happen in my case. Thanks in advance

Answer 1

I am not completely sure what line the error message is indicating, but the following loop is the only place I can think of a multiple response on your code
workflows.forEach(wf => {
//foreach is looping
if (wf) {
if (wf.workflowId === workflow.workflowId) {
res.status(409).send({
error: 'Workflow with that id already exists'
});
//but I don't think this guy will stop looping after the first "send()"
}
}
});