I've imported that very example into QB 2018 and it imported without issue. I also removed the first two lines and it imported that modified iif template into a new file and it also imported without issue. I could not see any difference between the two transactions.
I tried this same test in QB 2019 and it noticed the duplicate Vendor name and did not import.
As I understand it, the !VEND header is only necessary to add a new Vendor Name into QuickBooks and the iif transaction. If the names already exist in QuickBooks, you would not need any of the !VEND headers. Yet, since more than one name can be added from the template, my only guess is that the original template author placed both identical headers as a means to enter multiple names.
I am working with our developers to see how we can improve those templates or messaging to avoid any confusion as I was also perplexed by that specific template example.
Thanks for bringing it to our attention. ~B