Can anyone tell me what is double spend and can we use it to send money twice
A successful double spends means that both of your transaction will be confirmed. This would not happen with Bitcoin due to the way it is designed, blocks ensure that no transaction with same inputs can be included twice. However, it is possible when accepting unconfirmed transaction, when there is a fork and an attacker having 51% of the hashrate (They generate blocks faster than others, hence being able to change the blockchain after the last checkpoint). Generally, the more confirmation it has, the harder it is to reverse it.
