My wife recently subscribed to Spotify Duo and sent me a link to activate the second account. I did this and logged in on my phone without issue.
I have tried to log in to this account on my desktop but it appears a second account was created when I followed the link from my email (i.e. I already had a free account linked to the email address, the Duo account is using a slightly different username). When I try resetting the password on the Duo account I am not receiving an email link. If I enter the email address rather than the username I get a password reset link for the free account.