<p>It can only exist in one block.  That is why it is safer to wait for 2-6 confirmations before accepting a large transaction.  </p>

<p>So to answer the title of your post - a miner can't include a transaction which exists on another block in the same chain since it would not be a valid block.</p>

CC BY-SA 4.0
2019-02-14T20:51:02.363Z
thanks, who check these blocks for 2-6 confirmations?
monkeyUser
CC BY-SA 4.0
2019-02-15T11:07:28.997Z
@monkeyUser: the anxious recipient
RedGrittyBrick
CC BY-SA 4.0
2019-02-15T21:12:30.303Z
@monkeyUser It's up to the person receiving the Bitcoin to check the number of confirmations.  If you own a Lamborghini dealership an a customer pays with BTC, after 1 confirmation you let the customer drive off.  The chain re-organizes and that BTC is no longer valid, and you don't have the money in your account.  You should have waited for 6 confirmations before letting the customer drive off!
Zelgada
CC BY-SA 4.0
2019-02-14T20:17:47.557Z
2019-02-14T20:17:47.557Z
Zelgada
<p>Suppose that I have a Transaction A. (txA)
I send my txA to bitcoin network and full node validate it and put in Mempool and starts to broadcasting it.</p>

<p>A Miner A connect to a fullnode to create a candidate block and wins the POW. In this block there is my txA</p>

<p>A miners B create another block with all different transactions except to my txA.</p>

<p>If block A and block B have the same previous block Hash it's a fork,and if the chain of block A wins, all transaction of block B return to mempool.</p>

<p>But if B Has the previous block hash of A ?
After create a block there is another check of all transactions?
When TxA is in mempool it's unconfirmed when become confirmed? </p>

<transactions><blockchain><mining-theory><block><blockchain-fork>
what happens if a miner include a transaction already exists in another block?
monkeyUser
# what happens if a miner include a transaction already exists in another block?

(Asked by: username_0 on 2019-02-14)

Suppose that I have a Transaction A. (txA)
I send my txA to bitcoin network and full node validate it and put in Mempool and starts to broadcasting it.

A Miner A connect to a fullnode to create a candidate block and wins the POW. In this block there is my txA

A miners B create another block with all different transactions except to my txA.

If block A and block B have the same previous block Hash it's a fork,and if the chain of block A wins, all transaction of block B return to mempool.

But if B Has the previous block hash of A ?
After create a block there is another check of all transactions?
When TxA is in mempool it's unconfirmed when become confirmed?

**Tags:** transactions, blockchain, mining-theory, block, blockchain-fork


---

## 1 answers

### Answer by username_1 on 2019-02-14. Score: 0 | Accepted answer

It can only exist in one block. That is why it is safer to wait for 2-6 confirmations before accepting a large transaction.

So to answer the title of your post - a miner can't include a transaction which exists on another block in the same chain since it would not be a valid block.

> **Comments:**
> **username_0**: thanks, who check these blocks for 2-6 confirmations?  
> **username_2**: @username_0: the anxious recipient  
> **username_1**: @username_0 It's up to the person receiving the Bitcoin to check the number of confirmations.  If you own a Lamborghini dealership an a customer pays with BTC, after 1 confirmation you let the customer drive off.  The chain re-organizes and that BTC is no longer valid, and you don't have the money in your account.  You should have waited for 6 confirmations before letting the customer drive off!  

---

