I understand that it is disappointing to miss out on a great bargain, but it is a case of first come, first serve.
How do you know that the people that managed to buy the tablets were "non" Lidl shoppers? And why should that make any difference to you? Would you be less upset if regular shoppers had got them and you still had missed out?
I love Lidl and Aldi, great bargains but I accept that they have limited stocks of most things and if something isn't available when I'm shopping, it's just tough I'm afraid.
If you want a supermarket with better reliability of stock, go to Tesco, Asda, Sainsbury's etc.