combine multiple values of fields in sqlite database

I have a table that has 3 columns (ID, key, value):

key has repeated text, but value does not.

How do I combine/append the texts of value into one field according to key, separating them with a comma or newline, and delete the repeated texts of key?

If the order of the values in the results is not important, then you can do it with aggregation and GROUP_CONCAT(), which in SQLite does not support an ORDER BY clause and the order of the values is not guaranteed:

SELECT MIN(ID) ID, key, GROUP_CONCAT(value) value 
FROM tablename 
GROUP BY key

If you need the values ordered by ID, then use GROUP_CONCAT() window function:

SELECT DISTINCT
       MIN(ID) OVER (PARTITION BY key) ID, 
       key, 
       GROUP_CONCAT(value) OVER (
         PARTITION BY key 
         ORDER BY ID
         ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
       ) value
FROM tablename