Double check-out buttons is the most problem found in WooCommerce sites the simple reason is due to duplicate pages, checkout step is used in WooCommerce plugins and the same is used in theme WooCommerce folder.
To fix this issue simple tweak is to rename OR delete folder to do that go to wp-content >> plugins >> WooCommerce >> templates & Rename folder Checkout (Make sure you are not deleting the folder as if something went wrong you can make it back to original). Once that done try checking the site for duplication. Do not rename Checkout folder located under your theme folder as if you do then it might affect your ordering system. This is common fixture for the issue Double Check-out Buttons on Cart Page and Duplicate Fields
If still not get fixed then contact theme support to get it sorted.
If the issue fixed and some other issue is occurring let me know via comment system and will help you to get it sorted.