what happens if two people send the same amount to double the one before them?
What do you mean exactly?
If three entries are for example:
1
2
2
the first one gets 2 first, the second one has to wait on 2 more (because he should get 4). Or:
1
1
1
first one gets 2, the second and third have to wait.
Does this explain it to you?
It simply shifts to the next if there is enough money to pay the previous double the amount back.