well, random is random so duplicates may happen.
In your case a 1 in 16 chance.
korbaach knows a better way I am sure, (please let us know Korbaach )
one way is
I like using sprites as it is more visual for me...
be sure to change the count to however many sprite2 you have on your layout
remember even in this example above, when the count resets to zero, there is a chance (1 in 16 your game) it may reset to the same sprite.