MySQL - Duplicate entry error when trying to add new column

Question

I have a MySQL database with a table that has 2 million rows using innodb engine. I want to add another column, but I keep getting the following error:

Error 1062: Duplicate entry '' for key 'PRIMARY' SQL Statement: ALTER TABLE `mydb`.`table`  ADD COLUMN `country` VARCHAR(35) NULL DEFAULT NULL AFTER `email` 

How can I add the column without getting this error?

EDIT: Table definition

id int(11) NOT NULL AUTO_INCREMENT,   
user_id varchar(45) NOT NULL,   
first_name varchar(150) DEFAULT NULL,   
last_name varchar(150) DEFAULT NULL,   
gender varchar(10) DEFAULT NULL,   
email varchar(100) DEFAULT NULL,   
created_at bigint(20) DEFAULT NULL,   
updated_at bigint(20) DEFAULT NULL,  
PRIMARY KEY (`id`,`user_id`),   
UNIQUE KEY `user_id_UNIQUE` (`user_id`),   
KEY `first_name` (`first_name`),   
KEY `last_name` (`last_name`)

EDIT #2: SHOW INDEXES output

Table       Non_unique  Key_name        Seq_in_index  Column_name     Collation  Cardinality Index_type
table       0           PRIMARY         1             id              A          3516446     BTREE      
table       0           PRIMARY         2             user_id         A          3516446     BTREE
table       0           user_id_UNIQUE  1             user_id         A          3516446     BTREE
table       1           first_name      1             first_name      A          390716      BTREE  
table       1           last_name       1             last_name       A          439555      BTREE

Answer 1

it solution will lock table on write, but often suitable for solving the problem if table is not very big

LOCK TABLES my_table WRITE;

ALTER TABLE my_table 
ADD COLUMN `ts` DATETIME NULL AFTER `id`;

UNLOCK TABLES;

Answer 2

As described in the documentation, When running an online ALTER TABLE operation:

... the thread that runs the ALTER TABLE operation will apply an “online log” of DML operations that were run concurrently on the same table from other connection threads. When the DML operations are applied, it is possible to encounter a duplicate key entry error (ERROR 1062 (23000): Duplicate entry), even if the duplicate entry is only temporary and would be reverted by a later entry in the “online log”. This is similar to the idea of a foreign key constraint check in InnoDB in which constraints must hold during a transaction.