MacBook Double Login

I'm glad you decided to perform a clean install even if the previous owner may have already done so. It is always best to be sure.

Is "User 1" an account you created or is this a left over from the previous owner? When you reach the second login with another set of users did you also create these user accounts? If you did not create all these user accounts (on either login screen), then I suggest performing another clean install of macOS assuming you have a backup of your data.

Did you erase the whole physical drive when you performed the clean install previously? If not, then I suggest you do so now. If you are installing macOS 10.13+ you will need to click "View" within Disk Utility and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. Erase the physical drive as GUID partition and APFS (top option). This only works with an Intel Mac.

If you don't have a backup of your data, then try resetting your password using the instructions in this Apple article although I'm not sure how relevant it is in your situation due to your user account login being split from the Filevault unlock.

https://support.apple.com/en-us/HT202860

If you enabled Filevault and later changed your user account password, then sometimes Filevault may continue to use the older password. It is possible to re-sync the user account password and the Filevault password again if you can log in. Once logged in go to the "Security & Privacy" pane in System Preferences and select the "Change Password" option. Changing the password here will once again link the Filevault with your user account so only one password needs to be entered to log in. It will allow you to use the same password if you wish.