website page URL problem

hi, I just noticed a bit strange problem with my as well as other WP sites does that same.

1. mysite.com/chaild-page
2. mysite.com/chaild-page/123/

both of the above url returns the same page, however, No. 2 page does not exists.
to me No.2 suppose to be the 123rd page in case of pagination however, there is no pagination its a page not a post either.
incase of 123 I can put any number and same page comes up. however, I can not put a character. incase of character it returns 404 which I believe should be 404 in case of numbers.
please see a real example below:

example 1. https://wordpress.stackexchange.com/questions/105977/wordpress-pagination-not-working-always-showing-first-pages-content/

example 2. https://wordpress.stackexchange.com/questions/105977/wordpress-pagination-not-working-always-showing-first-pages-content/2/

the URL in example 2 should not exists.

how do I get rid of no2. URL in my site?
I want that URL to return 404
As now google is indexing the page separately as duplicate content.
Thanks