While technically accurate, this is non-standard to such a degree, and represents such a minority percentage of the population, that "don't do this" really is an appropriate response to the user.
I'm curious how often people share e-mail addresses. I used to see it a lot for people who used their ISP's mail system. Most people who are even slightly tech oriented have gmail, hotmail, etc these days, but now I can't help but wonder how many people just stick with the default ISP e-mail.
Mailing lists and shared mailboxes might be used by several people (support@foo.com). Noone uses them to sign up to online services I guess, but the address might be stored in a database and be assocoated with several people somehow.
True people do share. But you have to believe it isn’t. If your system already assumes email must be unique, then your system will make email field as a unique constraint. Otherwise what will you considered as unique?
Phone number, however, should be avoided as an ID. Far more likely to have phone number ownership changed than Google reclaiming your email address and give it to someone else realistically.
If your system already assumes email must be unique...
...then you're not building a system that works with the way users work. Some (very small) percentage of your users will have problems.
I first encountered this problem when I asked my parents to test my first startup. They shared an email account. When they signed up to things they just used the shared account. It was fine until they wanted separate accounts on the same service and it failed. It's not OK to assume everyone has a unique email address and that works as a reference to an individual. The assumption is incorrect. Email addresses are not unique to individuals.
A lot of services fail at this. If you're targeting older people then you shouldn't use a users email address as a username, and nor should you use it as a security factor.
> It was fine until they wanted separate accounts on the same service and it failed.
When it comes to designing a system, I try my best to prioritize this way: security, privacy, and finally usability. A product with the best user-experience may have the most vulnerable design. I say this because we need to balance between security and UX. Pick some constraints which make sense to your system.
If we have to accommodate every corner case, the system will become extremely insecure and unreasonable. In your case, perhaps solve it with "sub-account" if that makes sense for your system. To Netflix, sub-account seems reasonable so Netflix gives that as an option for user to share his/her account with his/her family/buddies.
Actually, sorry if being a little out of bound, you probably should help your parents setting up unique email address for them. They want a separate account for some specific reasons, I assume, so why not help them?
Email was created TO be unique. It is we the users who decide how to use that email address, BUT it is not the system/service designer TO accommodate every use case.
They want a separate account for some specific reasons, I assume, so why not help them?
That's the point - they didn't want different email accounts. They wanted to share one. There will always be edge cases that don't conform to your expectations of how users should be doing things, and your applications probably need to work even in those less-than-ideal circumstances. If your app is going to be used by older people who might share an email address then I wouldn't make the account process require a unique address for every user.
I actually go a step further these days and try to make things that don't even require any email address. The less personal information required to create an account the better. So long as users are aware that they won't be able to recover an account if they forget their password there's no real need to ask for an email in the first place.
How did they want to identify themselves upon using the service? Email plus name? Is it worth making login more complex for everyone else?
Or do you expect old people that share an email account to gracefully handle unique usernames? I'm pretty skeptical of that one.
And "So long as users are aware that they won't be able to recover an account" sounds like a joke. Maybe one in twenty people will truly internalize that, if you're lucky.
