If there are two arguments to separate users, uid (is an anonymous cookie) and user_id (is a logged in user), but our users can have multiple accounts and one uid can have more than one user_id, right now we just delete those users, but maybe there is some solutions to leave them in the same group?

I'm not entirely sure that I understand. It is very common to have both an anonymous id and user id. You can add these identities to the datasource and then create a join table so that you can merge them together during analysis: https://docs.growthbook.io/app/datasources#identifier-join-tables

if i use only user_id, then users who have multiple accounts can be bucketed in all versions of experiment, and i know it when check uid (cookie)

How do you know they have multiple accounts? Is there another id?

yes, one cookie (uid) have multiple user_id, and one user_id may have multiple cookies over time

Right so like someone signed in on two different devices