What is Double Spending and How it can happen?
Since when we send the BTC payment it deduct the BTC amount from the balance. If so how it can happen?
It is a type of exploit. The person sends funds without a transaction fee, then sends again with a fee. The transaction with the fee will get confirmed and the other will eventually get rejected by the network. It can also happen if a mining pook has then 51% of the global hashrate. They would technically be able to chose which transaction were confirmed and which weren't.
Not all miner will confirm the one with the fee instead of the one without the fee. Some nodes choose to just relay whichever transaction came first and if the transaction with the same UXTO appears, it will not be included in mempool.