I kind of knew/suspected the email would be the common data so it's not really a problem doing so but does it really matter? Is it needed or do most people just

I kind of knew/suspected the email would be the common data so it's not really a problem doing so but does it really matter? Is it needed or do most people just sign in with one site and stick to that? I was actually more wondering what is common practice, i don't have any popular sites requiring logins myself so i was wondering what is the norm. Do your system link multiple accounts with the same email then?