Why did my subscribers get the same email twice?

Four ways this genuinely happens

None of these is a fault. Each one is a systeme.io feature working exactly as documented, which is why the same content can land twice with nothing wrong on your account.

A deactivated campaign email was switched back on

Turning off every email in a campaign, then turning them back on, resends all of them to everyone still enrolled, including contacts who already read them the first time. systeme.io's own advice, given for this exact situation, is to build a new campaign rather than reactivate emails for people already inside one.

Someone pressed resend to non openers

A newsletter carries a Resend to contacts who did not open option, reached from the three dots beside it in Emails, then Newsletters. It duplicates the newsletter, lets you keep or change the subject line, and sends that copy again, only to the contacts systeme.io recorded as not opening the first one.

systeme.io caps this at one attempt: a newsletter can be resent once, and a contact who still does not open the second copy is not sent a third. If a subject line change went out this way recently, this is very likely the explanation.

An automation rule fired again for the same contact

A trigger can fire up to 20 times for the same contact before systeme.io deactivates it for that person. Only one trigger, Email link clicked, carries a built-in cooldown, one minute, to stop it firing twice from an accidental double click. Nothing equivalent is documented for the others.

So a rule built on Tag added or New sale can genuinely fire twice for one contact: a tag removed and reapplied, or a second purchase, sets it off again. If the action attached is Send email, that email goes out again too.

Someone was subscribed to a campaign they were already in

Subscribing a contact to a campaign by hand asks which step they should start at. Pick the first step for someone who already went through the whole campaign once, and email one goes out to them again, in full.

What none of this is

It is not systeme.io sending duplicates on its own. None of the help articles describe a technical fault that resends an email without a cause behind it. Each of the four above needs somebody, or some rule, to have actually done something. Checking Emails, then Campaigns for a recently reactivated email, and Automations, then Rules for a trigger with no cooldown on it, accounts for most cases.