Hi,
Go in the menu Customers>Email history to check the emails being sent.
There, you can see if HikaShop sends duplicates or if it's at the email server level that the emails get duplicated (in which case you'll want to contact your hosting or your server administrator).
You can also check there if there is a BCC or CC added to the emails, which could link to the email being received several times if the customer email address is the same than the email address that was entered in the BCC or CC fields of the email, which you can configure via the menu System>Emails.