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I am trying to access my 2018 tax return. It doesn't show up in my TurboTax account, despite the fact that I used the same account from last year.  When I use the TurboTax Assistant "Access a prior year tax return" option, it says "Hmm… I couldn’t find a return for that year in this account. However, I did find another account under your email. Your return might be there. Sign out in the bottom left of your screen, then sign in to that account to find out."

However, my email last year and this year is the same. How can I have two accounts linked to one email?

You can have 5 accounts for 1 email address.  Did you use this article?  That's common to end up with multiple accounts. First LOG OUT of whatever TurboTax account you're logged into right now. Then use this TurboTax account recovery website to get a list of user ID's for an email address. Run the tool against any email addresses you may have used
https://myturbotax.intuit.com/account-recovery/

If you used the Desktop CD/Download program then the only copy is on your computer and not saved or stored online.

When I go to that address, and type in my email, I don't see a list of user IDs, I get a "Let's make sure it's you To reset your password verify your info. This helps keep your account safe." screen. I have the option to verify with phone or email. I went through the steps with email, and it just made me reset my password.

No page where multiple account names are displayed. This is the same email address that I have a TurboTax email from stating TurboTax e-filed my 2018 return.