Shipping confirmation mail is sent to multiple customers

Magento 2.3.5-p2

This is whats happening:

1. shipment is created and shipment mail is sent to [email protected]
2. mail is sent to [email protected] again and also to [email protected]

The shipments for the 2 were created close together, so maybe thats related. I disabled async mail sending already – did not help. Looking for the problem it seems it has been a problem in Magento 1 as well. Nothing for Magento 2 though …

Any idea how to fix that?

Thanks