The possible problem

if your checkout button appears twice after a Woocommerce plugin update, one problem could be that your form-checkout.php file is not compatible with the latest version.

 

The fix

For example it could be located in your themes folder in a template override structure such as explained here 

 

A template override location would look something like

yoursite/wp-content/themes/your-theme/woocommerce

 

form-checkout.php  would then be  located in:

 <span style="color: #993300;">yoursite/wp-content/themes/your-theme/woocommerce/checkout/form-checkout.php 

 

Make a backup of the file, and then insert the file from the woocommerce plugins folder, overwriding your file in the above mentioned location.

The file you need is located here:

yoursite/wp-content/plugins/woocommerce/templates/checkout/form-checkout.php