Another Apple user is using my email address as his Apple ID email and i want to get it back

The question is: how did he assign my email as his Apple ID and how to reset such assignment?

We don't know, nobody here kown who he is.

For more support and assistance see -> Contact Apple for help with Apple ID account security

Contact Apple for help with Apple ID account security - Apple Support

but i found this email already in use by another user

Who is the other user?

You are sign in here .  .  .  with your Apple ID & Password.

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

You can’t merge multiple Apple ID accounts.

What happens when you use the same Apple ID & Password that you use to sign in here?

See -> Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support

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See also the following Apple articles:

If you think your Apple ID has been compromised

If you think your Apple ID has been compromised - Apple Support

Recognize and avoid phishing messages, phony support calls, and other scams

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Report a security or privacy vulnerability

Report a security or privacy vulnerability - Apple Support

Get help with security issues

Get help with security issues - Apple Support                                         

Never jailbroken your iPhone.

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If your Apple ID is locked or disabled -> If your Apple ID is locked or disabled - Apple Support (CA)

-> Get a verification code and sign in with two-factor authentication

Get a verification code and sign in with two-factor authentication - Apple Support

For more support and assistance see -> Contact Apple for help with Apple ID account security

Contact Apple for help with Apple ID account security - Apple Support