There is an observer for passing custom parameters to invoice template. Try the below one in your custom module.
<observer name="vendor-add-custom-field-data-invoice" instance="Vendor\Module\Model\Observer\Invoice\EmailTemplateVars"/>
in the config.xml file you need to add the below code in tag
I had the same mistake.
In my case it was due to the switch to PayPal Plus, which caused another webhook. And then two confirmations were sent by PayPal.
I simply deleted the second webhook and since then only one confirmation is sent and thus created only one invoice.
The webhooks can be found at http://developer.paypal.com/ on "MyApps & Credentials" ...
I have same problem,
and i found this on my log:
[2019-08-19 20:27:23] main.INFO: Add of item with id WeltPixel_GoogleTagManager::GoogleTagManager was processed  
[2019-08-19 20:27:23] main.DEBUG: Request validation failed for action "Magento\Sales\Controller\Adminhtml\Invoice\View\Interceptor"  
[2019-08-19 20:27:23] main.DEBUG: Request validation ...