First button click- got a list from database table [1, 6, 7, 8]

Now i have to add this in a blank list.So as the first button is clicked it should give you the same list [[1, 6, 7, 8]] because there is a only one list to add.

and i am getting [[1, 6, 7, 8]]

second button click- got a list from database [1, 2, 5, 6, 7, 8]

Now second button is clicked so i should get [[1, 6, 7, 8],[1, 2, 5, 6, 7, 8]]

but what i am getting is [[1, 2, 5, 6, 7, 8]]

Third button click- got a list [2, 4, 7, 8]

after clicking third button [[1, 6, 7, 8],[1, 2, 5, 6, 7, 8],[2, 4, 7, 8]]

but i am getting [[2, 4, 7, 8]]

And so on depending on the number of buttons selected by user,which we don't how many buttons he can select.

Output after applying intersection function [7,8]