If you want to use the same email address for a second account, you'll have to make a small addition to your email address on the signup form.
A plus sign (+) and a keyword need to be added before the "@" sign of your email address.
For example, if your email address is email@example.com, you can create multiple accounts using keywords such as:
The keyword you use doesn't matter. It's just to help you keep track of which account is which.
All emails for these accounts will still be sent to the inbox of firstname.lastname@example.org.
The "+keyword" is just a tool that many (but not all) email providers offer their users as a neat way to sort incoming emails. Check the support documentation of your email provider to see if they support this feature. You can also test if they support it by sending a test email to your "+keyword" email address.
Pro-tip: You can use "+keyword" in other areas of your online life. For example, if you sign up for an online newsletter, you can signup using satoshi+newsletterXYZ@protonmail.com. If you start receiving emails addressed to that address from other senders, you'll know who sold/shared your information.
Security note: remember to use Two-Factor Authentication (2FA) on your Kraken accounts as well as on your email address. If you have multiple accounts tied to a single email address, that's double the damage if your email address is comprised due to a lack of 2FA.