If the target product in your upsell offer is being added to cart twice, this is because your store ajax cart setting does not match the setting in the detect shop ajax cart section of the app.
To fix this, please go to the app's dashboard >> Settings tab >> Other Settings and follow as instructed below:
For stores using ajax cart: choose Auto or Shop is using ajax cart. First you can test the Auto and if it's not helpful, please change to Shop is using ajax cart, then use an incognito window to test again.
For stores not using ajax cart: choose Shop is not using ajax cart.
Don't hesitate to contact us at firstname.lastname@example.org if you need further assistance! Boost your revenue now with Boost Upsell.