Hate it how people actually think you're going to get the same world twice with seeds.
RANDOM world generator.
RANDOM
Computers can NOT be random. At all.
To make it appear AS IF they are capable of doing something random they take a number (like the amount of seconds that have passed since 00:00:01 January 1st 2000) and then apply a series of mathematical formulas to get the desired (seemingly) random result. The numeric value that is input at the start of such a program is known as the SEED. Therefore if you input the same seed into the same program twice, you are going to get the same result twice.