如何在不覆盖的情况下将2个表组合成第三个表SQL？

so i have 3 Sql tables, I run this command

INSERT INTO `wp_atkv_EWD_OTP_Customers` (`Customer_ID`,`Customer_Email`,`Customer_Created`) 
SELECT 
    `User_ID`, 
    `Username` , 
    `User_Date_Created` 
FROM `wp_atkv_EWD_FEUP_Users`

and it works perfectly fine I want to add data from another table using the same user id but then I run the command below it says duplicate error? Should I not be using insert into? also if i ignore the duplicate it just creates new entries I want them all in the same row

INSERT INTO `wp_atkv_EWD_OTP_Customers` (`Customer_ID`,`Customer_Name`) 
SELECT 
    `User_ID`, 
    group_concat(`Field_Value`) 
FROM `wp_atkv_EWD_FEUP_User_Fields` 
GROUP by `User_ID`

What Should I do how can I update with the third table?

I'm expecting a join:

INSERT INTO wp_atkv_EWD_OTP_Customers (Customer_ID, Customer_Email, Customer_Created, Customer_Name)
    SELECT u.User_ID, u.Username, u.User_Date_Created, uf.vals
    FROM wp_atkv_EWD_FEUP_Users u LEFT JOIN
         (SELECT User_ID, group_concat(Field_Value) as vals 
          FROM wp_atkv_EWD_FEUP_User_Fields
          GROUP by User_ID
         ) uf
         ON u.user_id = uf.user_id;

Alternatively, you might want an update:

update wp_atkv_EWD_OTP_Customers c join
       (select `User_ID`, group_concat(`Field_Value`) as vals
        from `wp_atkv_EWD_FEUP_User_Fields` 
        group by `User_ID`
       ) uf
       on c.customer_id_id = cf.user_id
    set Customer_Name = uf.vals;