C++ problem

A children's game of matching figures, to develop memory, consists of searching, in a group of hidden cards, for two cards with the same values. Each time a pair is found, the two cards are revealed.

Write a program that randomly fills a 6x6 matrix with 18 pairs of numbers (from 1 to 18) to play this game.

an example of a matrix generated by the 18 pairs: :

The matrix

must have only 2 repeated values