The problems
Someone bought. They still got the cart email.
Shopify Email and Klaviyo both think they own the cart. That's a store problem, not a subject line. Email marketing is an offer: we fix the senders, or a retainer if you want us finding the next leak.
How to stop Shopify Email and Klaviyo both sending
You run the brand. A cart email after they buy is one leak on the storefront. If that's the one costing money, we start here.
A duplicate abandoned cart is Shopify Email and Klaviyo both sending the same cart. Shopify Email can send abandoned checkout by itself. Klaviyo sends the flow you built. If nobody turned Shopify Email off, both fire. Sometimes a third note from helpdesk. The Klaviyo shop is asked to fix the flow. They change the subject line. Shopify Email is still on.
Added to Cart, Started Checkout, and Abandoned Checkout are not the same thing. Themes mix them up. A leftover snippet fires the same event twice. Someone already bought and still gets the cart series. That's not copy. That's the store talking twice, or talking wrong.
Duplicate onsite.js is the other classic. App embed on. Old snippet still in the theme. Each one can emit the same event. "Klaviyo is set up." It's set up twice.
Why the usual vendor misses it
A Klaviyo shop lives in Klaviyo. They will not open the theme. They will not turn off Shopify Email. A freelancer will close the ticket: "turned off Shopify notifications." They will not hunt leftover snippets, and they will not still be here after the next theme publish.
The leak is the senders and the leftover script. That's the job.
What we do on the store
We look at who can email a cart: Shopify Email, Klaviyo, helpdesk. One story. The others off.
We look at what's listening in the theme: app embed, old snippet, custom pixel. One Klaviyo path. Leftovers gone.
We check that the events match what happened. Cart means cart. Checkout means checkout. A purchase should not get another cart email. A refund should not get the "thanks for your order" flow. If the event is wrong, turning off Shopify Email is not enough. That work is on Shopify Klaviyo events.
We are not a Klaviyo agency. Email marketing is an offer. We can write the flows and campaigns once the store is telling the truth.
Next step
The storefront scan cannot see Klaviyo. That's on purpose. It can show leftover scripts and a heavy page. Duplicate carts usually need the theme after that. Email marketing is an offer: we fix the senders, or a retainer if you want us finding the next leak.
For guidance on which email platform to use at your revenue band (Shopify Email vs Klaviyo), see the email systems stack. This page is store-side duplicate sends. That page is which tool.
Straight answers.
Shopify Email can still send abandoned checkout. Klaviyo sends the flow you paid for. If both are on, the same cart gets two emails. Turning one off is step one. A leftover Klaviyo script in the theme can still fire twice after that.