Hello @juliangil,
I am suspecting that there is something inside your theme that is causing this to duplicate. I would look to see if your theme needs updating, (of course, backup your site first!) and see if that resolves the issue.
If it doesn’t, the next step would be to temporarily switch to a default theme, such as Storefront or Twenty Seventeen and see if that resolves the issue, If it does, that means it’s the theme. If it does not, then next we’d want to check for plugin conflicts.
In order to test for plugin conflicts, you’ll want to disable all plugins except for WooCommerce and PayPal Powered by Braintree and see if that resolves the duplicated buttons. If it does, you’ll want to turn on your plugins, one by one, and see which one causes the buttons to duplicate.
Hopefully that can get you started on troubleshooting.