Check for duplicates in a randomly generated list and replace them

I am making a minesweeper game with randomly generated bombs. Yet at times I have found that there are duplicates in my list of coordinates of bombs. How do I check for duplicates in a list and replace them with other randomised coordinates.

from random import randint

def create_bombpos():
    global BOMBS, NUM_BOMBS, GRID_TILES
    for i in range(0, NUM_BOMBS):
        x = randint(1, GRID_TILES)
        y = randint(1, GRID_TILES)
        BOMBS.append((x, y))
    print(BOMBS)

The user can decide how big the board is by input of GRID_TILES. If they input 5, the board will be 5x5. The ammount of bombs is:

GRID_TILES * GRIDTILES / 5