problem description

in the table, id is the primary key, created_at is the timestamp, and the created_at of the data to be queried is 1538125292

first page:
SELECT * FROM 4s_active_slotmachine_import_player WHERE ( area_id = 18696) AND ( active_id = 6293) AND ( is_del = 0) ORDER BY created_at DESC LIMIT

Page 2:
SELECT * FROM 4s_active_slotmachine_import_player WHERE ( area_id = 18696) AND ( active_id = 6293) AND ( is_del = 0) ORDER BY created_at DESC LIMIT 20 OFFSET 20

the environmental background of the problems and what methods you have tried

it is found that the second page shows the data that appeared in the first page, and it is no problem if the query does not add order by, or it can be sorted with id.
sorting with created_at of the same value will result in the same data appearing on multiple pages.

what is this?