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What is Double Spending?

By: Alexander Reed | Last updated: 1/6/24

One of the main issues any digital currency faces is double-spending. This post explains exactly what the double spending problem is and how to prevent it.

What is Double Spending Summary

Double spending is the act of trying to spend the same digital currency twice by creating duplicate transactions. This issue is one of the main obstacles a digital currency needs to solve in order to make sure it is not being abused and that it maintains its value and trust.

Bitcoin solves the double spending problem through the use of a public ledger that is constantly monitored by network participants and through the Proof of Work consensus mechanism.

That’s double spending in a nutshell. For a more detailed explanation, keep on reading. Here’s what I’ll cover:

  1. Double Spending Explained
  2. How to Prevent Double Spending
  3. RBF – “Legit” Double Spending
  4. Conclusion

1. Double Spending Explained

Since Bitcoin transactions are digital files, it’s actually possible to duplicate transactions and spend the same Bitcoin twice. This issue of “copying and pasting” is a weakness any digital currency faces (even fiat currencies in their digital form).

Here are some examples of how you can double spend a digital currency:

  1. You can “copy” a coin and send it to someone while still retaining the original.
  2. You can simultaneously send the same coin to two different people.
  3. You can reverse a transaction that’s already been made after receiving the goods paid by it, hence keeping the goods and the money.

The double spend problem is a serious issue for any digital currency since it can create an inflated money supply which quickly erodes the value of the currency and the trust in it.

2. How to Prevent Double Spending?

There are two ways to stop double spending – a centralized way and a decentralized way.

The Centralized Solution

The centralized solution to prevent double-spending is pretty simple. It usually involves a trusted authority that holds a record of everyone’s balance in the system.

For example, when Alice sends money to Bob, the transaction goes through the central authority (e.g., bank) that checks its ledger to make sure Alice has the money she wants to spend. If everything is in order, the central authority authorizes the transaction, and the money is transferred.

The Decentralized Solution

When you’re dealing with a decentralized currency like Bitcoin, you don’t have a central authority to validate transactions. Therefore, Bitcoin uses a mix of elements to solve the double spending problem.

Preventing Fraudulent Transactions

First of all, the Bitcoin ledger of transactions, known as the Blockchain, is public and visible to all. Every Bitcoin transaction ever made and every balance of every address can be inspected by anyone.

This means that if Alice sends one Bitcoin to Bob, every computer that holds a copy of the blockchain (also known as a full node) will verify the history of Alice’s transactions to make sure she indeed has that one Bitcoin to spend.

If Alice tries to cheat and creates a Bitcoin from thin air, she will quickly be exposed by one of the many nodes validating transactions.

Preventing Simultaneous Transactions

But what if Alice sends the same coin simultaneously to two people?

Assuming 50% of the nodes received transaction A first and the other 50% received transaction B first. How do we know which transaction is valid, and which to discard? The answer is simple: The transaction that enters the ledger first will be the valid one.

That’s why it’s always recommended to wait for at least 1 confirmation before considering a Bitcoin transaction complete.

In order to decide the order of transactions, Bitcoin uses a consensus mechanism called Proof of Work (POW). This mechanism describes the rules of who gets to update the ledger of Bitcoin transactions.

This is important since when there’s no central authority, you need to have a consensus about who gets to make changes in the transaction ledger. The whole process of updating the ledger of transactions is known as Bitcoin mining.

However, what happens if the mining process somehow happens simultaneously for both transaction A and transaction B?

Let’s assume that two unrelated miners managed to update the ledger at exactly the same time, each one with a different transaction that uses the same source of funds.

In this case, we will have 2 branches of the blockchain (also known as a fork), and the next block of transactions to be mined will determine which previous block was valid.

If the next block, in some magical way, is also mined simultaneously, we will have to wait until the next block, and so on.

If you want to be completely sure your Bitcoin transaction won’t be reversed in such a rare case of coincidence, you should wait until 6 confirmations arrive for your transaction.

It’s highly unlikely (like super highly unlikely) that this fork will happen more than 6 times. So, in the end, we will have a clear winner, and only one transaction will be confirmed.

Preventing transaction reversal

There’s a special kind of double-spend attack that is called a 51% attack. This happens when a single entity gains over 50% of the network’s mining power and can effectively control which version of the ledger is the legitimate one.

In this case, the attacker can spend coins on one version, obtaining goods or services for this payment. Later on, the attacker creates a different version of the ledger where the original transaction doesn’t exist and retrieves the payment to his possession.

The only way to prevent a 51% attack is to make sure the network is decentralized enough and has enough computational power to make it basically impossible to amass more than 50% of its hash rate.

In Bitcoin’s case, a 51% is highly unlikely, but with smaller coins, this type of attack has happened.

3. RBF – “Legit” Double Spending

One form of “legit” double spending is Replace By Fee, or RBF for short.

RBF is a function embedded in certain Bitcoin wallets (e.g., Electrum) that allows you to rebroadcast a transaction that is still unconfirmed in order to get it confirmed faster.

In some cases, Bitcoin transactions are sent with a network fee that is too small to incentivize miners to pick them up. This can get your transaction stuck inside the mempool, waiting to be confirmed for a very long time.

RBF allows you to rebroadcast the transaction with a higher fee, effectively trying to double-spend your money, so the miners will pick up the new transaction and the old one will get canceled.

4. Conclusion

Double spending is a major issue that needs to be addressed when dealing with digital currencies. Fortunately, it seems that Satoshi Nakamoto, the inventor of Bitcoin, managed to build a mechanism to prevent it from happening even in a decentralized network.

You may still have some comments or questions. If so, make sure to leave them in the comment section below.

Having delved into futures trading in the past, my intrigue in financial, economic, and political affairs eventually led me to a striking realization: the current debt-based fiat system is fundamentally flawed. This revelation prompted me to explore alternative avenues, including investments in gold and, since early 2013, Bitcoin. While not extensively tech-savvy, I've immersed myself in Bitcoin through dedicated study, persistent questioning, hands-on experience with ecommerce and marketing ventures, and my stint as a journalist. Writing has always been a passion of mine, and presently, I'm focused on crafting informative guides to shed light on the myriad advantages of Bitcoin, aiming to empower others to navigate the dynamic realm of digital currencies.

View all Posts by Alexander Reed

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14 comments on “Double Spending”

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    1. Hi Neeraj, this transaction fee is just too small, it might never get confirmed. You can use a transaction accelerator to have it confirmed for an extra fee.

  1. I got an idea of double spending in Bitcoin-Blockchain but i have a scenario
    Scenario – Let say Alice has only 10 BTC to spend and buys a merchandise of 10 BTC from Bob and initiate the payment in Bitcoin. Merchandise doesn’t wait for the payment to confirm and send the merchandise to Alice.
    But At the same time Alice initiate the same 10 BTC payment to her own account (having different BTC address) and tricked the network. This is the same case as of double spending.
    I know only one transaction would be confirmed by the network but if that would be second one then in that case Bob has already shipped the merchandise but he will not receive the money.
    What should be done in this case ? Shall Bob wait till transaction gets confirmed ? As Transaction confirmation may take 10 min to 1 hr (average) ?
    Please help me understanding this scenario if this is valid.

  2. Hello,
    tnx i have a problem.

    someone sent me $2679 and it did not confirm then i sent it out $2794 to my client lbc wallet. i had $110 sent to me which did not confirm. then later i sent out $135 to a bitpie wallet.

    after some hrs someone sent me $6 it confirmed well. then also someone sent me $1000 which confirmed as well.

    but the issue here is this, the $2679 after two days of no confirmation vanished from my wallet history, the $2794 i sent to my client lb wallet also vanished. then the $135 i sent to bitpie wallet reversed back to me together with another $105 i dont know where it came from which showed up in my wallet.

    then the $110 a client sent to me confirmed.

    This transaction of $2679 and the $2794 which i received and sent out is showing double spent. its since friday and 4 days now the harsh is still showing its heading to the destination wallet but when u login wallet it never there again.

    now when i login to send out from my wallet i will be getting a warning that ” An output is already spent in …. showing harsh id of two transactions i sent which reversed ”.

    same thing its telling my supplier who sent me the $2679 when he wants to send it back cos d coins reversed to him.

    Please can you help advice on this as my client is on me and i do not know what to do or do we have to wait further because i still have money in my wallet but cant send it out any funds.

    waiting your reply.


    1. Zsofia - 99Bitcoins support

      Hi, you cannot resend a Bitcoin until it has been confirmed on the blockchain, otherwise it will be double spending as it just happened. He will be able to send once the first transaction has been confirmed, it might take couple of days if the fees were low if that was not already a double spending. Otherwise eventually the transaction will be cancelled once the transaction is dropped off by all miners and will be rejected on the blockchain, but also needs time. At this point I don’t think so that you have much choice, just wait.

  3. Hi, I had an issue two days ago sending BTC from green address wallet to another wallet. The transacition was doubled in the receiving wallet, one was confirmed and the another is pending. I’m just starting with cryptocoins and after readind this article I understand that it was my fault for not waiting fot all confirmation. And what can I do about that pending transaction in the another wallet, will it be eventually cancelled and how long does it takes? Thanks you very much for your help.

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