No-Duplicate Delivery Pipeline: Event Registration to Certificate

Send each new registrant one certificate, designed so a re-run never sends a duplicate.

The workflow checks your registration system for new paid records, generates a certificate for each, and emails it. Out of the box it delivers certificates, but the same structure works for any "do this once per record" job. (Engineers call this idempotent: it's designed so running it again doesn't repeat what already went out.)

It's built around the failures that cause double sends or lost sends: a run that repeats, a crash halfway through, a mail server error, and an API that keeps returning the same records. It's designed for a single n8n instance, not multi-worker queue mode.

• Registry. A durable list of every record it has seen, marked pending, sent, or failed. A record that's already listed is not sent again automatically.
• Write-ahead reservation. Each record is reserved before anything is sent, and a run only sends the records it reserved.
• Second duplicate guard. The sender re-checks the registry on its own before every email.
• No automatic retries. A failed send is recorded and reported, not silently tried again. You decide whether to resend.
• One alert per problem. You get one email for failed sends, and one for sends stuck mid-way, where the recipient may or may not have received it.
• Run cap. A limit on sends per run, so a bad data pull can't flood inboxes. The rest wait for the next run.

Features

• Connects to most records APIs (adapt the fetch step to your platform)
• Generic certificate you restyle with your own branding
• Schedule trigger, plus an optional webhook trigger
• Registry, duplicate guard, alerts, and run cap as above

What's included

• 3 workflow JSON files (watcher, sender, registry)
• README with setup, configuration, and test steps
• Docker self-hosting guide
• License for one business

Requirements

• Self-hosted n8n 2.x (tested on 2.42.6)
• An API that lists your registrations or records
• An SMTP mail account

License

One license covers one business, for unlimited use in its own operations. Setting it up for a client? You can, as long as that client's business has its own license. No reselling or redistributing the files. Free updates for 12 months from purchase. Full terms are in the license in the download.

FAQ

Will it ever send a duplicate?

It's designed so a re-run never sends one: every record is reserved before sending, and the sender checks again on its own. If a run dies between sending and recording, that record is flagged as stuck and you're told, rather than it being sent again.

Why doesn't it retry failed sends?

An automatic retry is how people get the same email twice. Failures are recorded and you get one alert, so a person decides.

Does it only work for certificates?

No. Certificates are the example. Swap the sender step for any once-per-record action.

Which event platforms does it work with?

Any that offers an API listing your records. You edit the fetch and mapping steps to match its fields.

Can I change the certificate design?

Yes. It's a generic template meant to be restyled.

What if a large batch comes in?

The run cap limits how many go out per run. The rest go out on the next runs.