user64515: The Mycelium Wallet has created a Bitcoin wallet for me, then I've written down a 12-word passphrase for the wallet backup. However it looks like a set of words (in the Mycelium) to generate these phrases is limited - at least I've got a passphrase with one word repeated two times.
How can I make sure this passphrase is unique in our world where more than 7 billion people live?

user979: (Some wallets that generate word lists for recovery also allow a user-created passphrase, so I'm using "recovery seed" to denote the word list.)
I'm not familiar with Mycelium specifically, but many wallets use a standard known as BIP39 to generate recovery seeds. That technique draws the words randomly from a list of 2048 words, with duplicates allowed. The order (permutation) of the words is significant. So the number of possible recovery seeds is 2048^12 (^ indicates exponentiation). That is 5.44451787E+39, which is 5,444,517,870,735,015,415,413,993,718,908,291,383,296.

user44137: The chances of someone "brute-forcing" or guessing your seed is extremely slim. There are more than 5 duodecillion possible combinations of twelve-word seeds. Just so you get an idea of how big that number is, it's more than 1 thousand million million million million million million possibilities.

64515: <p>The Mycelium Wallet has created a Bitcoin wallet for me, then I've written down a 12-word passphrase for the wallet backup. However it looks like a set of words (in the Mycelium) to generate these phrases is limited - at least I've got a passphrase with one word repeated two times.</p>

<p>How can I make sure this passphrase is unique in our world where more than 7 billion people live?</p>
 979: <p>(Some wallets that generate word lists for recovery also allow a user-created passphrase, so I'm using "recovery seed" to denote the word list.)</p>

<p>I'm not familiar with Mycelium specifically, but many wallets use a standard known as <a href="https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki" rel="nofollow noreferrer">BIP39</a> to generate recovery seeds. That technique draws the words randomly from a list of 2048 words, <a href="https://bitcoin.stackexchange.com/questions/59904/does-bip39-mnemonic-construction-avoid-repeating-words">with duplicates allowed</a>. The order (permutation) of the words is significant. So the number of possible recovery seeds is <a href="https://www.calculatorsoup.com/calculators/discretemathematics/permutationsreplacement.php" rel="nofollow noreferrer">2048^12</a> (^ indicates exponentiation). That is 5.44451787E+39, which is 5,444,517,870,735,015,415,413,993,718,908,291,383,296.</p>
 44137: <p>The chances of someone "brute-forcing" or guessing your seed is extremely slim. There are more than <code>5 duodecillion</code> possible combinations of twelve-word seeds. Just so you get an idea of how big that number is, it's more than <code>1 thousand million million million million million million</code> possibilities.</p>
