I had never made a similar mistake before. When I am doing a backup of my seed phrase, I do make two copies and make sure to rewrite them two times on the piece of paper before I safely hide them.
And I think, before you proceed on the wallet like in Electrum, it will ask again or verify the seed if you write it carefully, or you can try importing the seed again if you are going to get a similar wallet just to verify if you safely wrote them down.

That one missing word is easy to brute force; if you still had this seed, you could use the Finderouter tool to look for the missing word. Just provide the public key or an address of that wallet and add an asterisk under the missing word.

You can find the tool here: https://bt.irlbtc.com/view/5214021.0

I am very careful about the seed phase of the wallet, I once kept my seed phase online in a new state and it got hacked from there, I corrected myself from that mistake. I write it in a paper diary and check it seven times from the side and compare each letter, I don't want to lose confidence in myself in any way.
Maybe because of my little problem, I may have a problem with my wealth in the future, due to which I collect the seed phase very carefully and write it in the diary. And I keep it very safe and I have informed my very close people so that even after my death, the seed phase of my wallet is not lost.

I haven’t come across repeated words(seedphrase) whenever I’m creating a wallet.And I was curious when I read the Op’s content I had to carry out my own findings to figure out what the Op meant and when I did my findings I discovered that it is something that usually happens when creating an account in a wallet.But regardless of that,it’s crucial to cross check important data like seedphrase because your seedphrase is your recovery phrase so importantly it can help you recover your wallet when your device got stolen or missing.Above all nobody is above mistake,certain things happen for certain reasons if had written your seedphrase correctly you wouldn’t have discovered that a word can appear twice,so atleast your mistake have added knowledge you and the community.

I've had to use my seed phrase to recover my wallet only once(when I had to reinstall the Operating System on my laptop) and it worked fine.
AFAIK, almost all cold wallets have a built-in feature forcing the user to repeat the seed phrase right after the 12 words were created. This is to make sure you don't miss any of the words and you don't repeat words. So to answer your question, no I haven't made such mistake. There are other points of failure. I keep my see phrase on a piece of paper, so I might lose that piece of paper someday, or I might throw it away by mistake. You never know what is going to happen.

No, I've never made such mistakes because I'm very careful about them. I think all Bitcoiners fall into one of two categories: some keep their seed phrase online, while others write it down on a notebook. But I don't like the idea of someone finding the notebook, and then all your hard work accumulating Bitcoins would be for nothing. After all, anyone could enter the seed phrase, and you'd no longer own your crypto. I once thought it would be cool not to write down the last word of the seed phrase anywhere. Instead, I'd memorize it. Like the word "Play," for example. I think that's something you'll never forget.

A few times I've created wallet addresses, but I've never come across a seed that repeats exactly like yours. Is it possible for someone to have the same seed as you? I don't think so. But I have missed some seeds a few times when opening my wallet. It's definitely my own negligence. I wasn't focused enough when writing down the seed, so only 11 seeds were recorded. But I also sometimes forget what type of wallet I made, for example, Electrum has several types like Standard wallet, 2FA, or Multisig Wallet.

Double checks or more checking times when you write down a wallet seed phrase is good, but not enough. There is another step, that is important, practice of using your backup to recover that wallet, compare the original wallet and a recovered one to see whether they're the same.

More DOs and DON'Ts with wallet backups in this following article from Jameson Lopp, and more topics.
How to back up a seed phrase.
Crypto Security - Additional Protection For Your Seed/Private Keys.
Protect yourself from fake wallet software (guide).
[GUIDE] How to Create a Strong/Secure Password.

It's not only for offline, airgap backups but for digital wallet files you have, you must secure them well and that means you must use strong passwords for your wallet files.

It is most important to be very careful about the wallet, remember that this is where your future will be deposited, i.e. the amount of Bitcoin you will deposit depends on your seed phase collection.
So every person should pay serious attention to the wallet, a small mistake here is enough to lose your Bitcoin and it will never be possible to recover it. So give importance to the wallet and build your future beautifully.

With wallets, and backups, people must do it properly and safely. That means they have to back up their wallets, and test backups to check whether they actually backed it up accurately, and the backups are actually usable for recover wallets correctly.

This check backup step is important and is noted here.
How to back up a seed phrase?
About repeated words in a wallet seed phrase, Lopp has another article about it.
How Many Bitcoin Seed Phrases Are Only One Repeated Word?

I don't know why people don't do the obvious when they write down the mnemonic words backups: number each word properly. This also avoids later confusion in which direction you wrote down the words when you have multiple lines and/or rows (by line first, from left to right OR rows first, from top to bottom).

You should immediately spot when you don't have 12 or 24 words (most common number of them for standard BIP39 mnemonic recovery words).

It's not very likely that two adjacent words are the same, but not impossible. Each mnemonic word represents eleven bits of the starting random entropy (except for the last word which contains bits of a partial SHA256 checksum), so two chunks of eleven bits could be the same string of bits. Possible, but quite unlikely.

And someone else already wrote it and I find it quite puzzling, OP apparently didn't verify immediately after writing down the mnemonic recovery words, if he could successfully recover his wallet. Seriously, why no verification!? It makes no sense to skip proper verification.