I'm a bot author and I want to create a new bot account with the same e-mail address as my main lichess account. I am, after all, the author and operator of the bot, and I only use one e-mail address.
But lichess won't let me create a new account using the same e-mail address. The thinking seems to be that someone creating multiple accounts is probably up to no good. All you are doing, though, is encouraging those who really are up to no good to create a new account using a different e-mail address, making it less obvious that the two accounts belong to the same user.
To put it another way, if somebody is stupid enough to create a second account for nefarious purposes and yet use the same e-mail address, catching and dealing with them is easier than if they'd used a different e-mail address. Why make your job harder than it has to be?
(And yes, you can also catch it by watching for two users with the same IP address... but sometimes two accounts with the same IP really do belong to different people. Two accounts with the same IP and same e-mail probably do not.)
Aside from the bot account issue, there are other reasons for creating an alternate account that I consider legitimate. For example, I know multiple people who have a "sober account" and a "drunk account" to keep track of how well they play while sober vs. how well they play while drunk. I suppose you could argue you aren't allowed to have separate "sober" and "drunk" ratings in FIDE, but I think it's OK to take things a little less seriously than FIDE.
tl;dr: Disallowing multiple accounts with the same e-mail address only inconveniences those who would create multiple accounts for legitimate reasons, while making it harder to catch those who create them for illegitimate reasons.
But lichess won't let me create a new account using the same e-mail address. The thinking seems to be that someone creating multiple accounts is probably up to no good. All you are doing, though, is encouraging those who really are up to no good to create a new account using a different e-mail address, making it less obvious that the two accounts belong to the same user.
To put it another way, if somebody is stupid enough to create a second account for nefarious purposes and yet use the same e-mail address, catching and dealing with them is easier than if they'd used a different e-mail address. Why make your job harder than it has to be?
(And yes, you can also catch it by watching for two users with the same IP address... but sometimes two accounts with the same IP really do belong to different people. Two accounts with the same IP and same e-mail probably do not.)
Aside from the bot account issue, there are other reasons for creating an alternate account that I consider legitimate. For example, I know multiple people who have a "sober account" and a "drunk account" to keep track of how well they play while sober vs. how well they play while drunk. I suppose you could argue you aren't allowed to have separate "sober" and "drunk" ratings in FIDE, but I think it's OK to take things a little less seriously than FIDE.
tl;dr: Disallowing multiple accounts with the same e-mail address only inconveniences those who would create multiple accounts for legitimate reasons, while making it harder to catch those who create them for illegitimate reasons.