How to merge two MySql tables that have the same structure and were the primary keys of the two tables will clash.
One option is:
INSERT IGNORE INTO table_1 SELECT * FROM table_2;
which allows those rows in table_1 to supersede those in table_2 that have a matching primary key, while still inserting rows with new primary keys.
REPLACE INTO table_1 SELECT * FROM table_2;
will update those rows already in table_1 with the corresponding row from table_2, while inserting rows with new primary keys.