I'm using Zen Cart 1.5.4 and IntegratedCOWOA_2.4
The way it works on my site is that if you checked out with COWOA previously and you attempt to login using that email address the site tells you it has no record of that email address.
Then if you use either the COWOA or standard account creation process it allows you to create another 'profile' using that email address.
These two paths make sense since it is supposed to be 'Checkout Without a Profile / Account', not 'Checkout Without a Password' which is what COWOA was in the old version. I swear I read somewhere that the paths I described are the way it should work and it was done that way on purpose.
I may be wrong, but I think the reason yours works a different way is due to the modifications you mentioned in post #413 and that you and DivaVocals have discussed in regards to 'Duplicate Accounts via COWOA'
I don't think those modifications were ever officially integrated. On a couple posts after #413 it looks like DivaVocals built them into a Beta version 2.5 and 2.6, but the version on Zen-Cart.com is still 2.4
Maybe I'll pull the GitHub code and attempt an installation on my dev sit to see if I can get it to work.
One side question though: There is mention of some type of email issue where due to the multiple accounts, after the fix, customers might get an email to each separate COWOA account they created. Is that only if I was trying to communicate using the tools like Product Notification Manager?