By a somewhat generous interpretation classical computer memory depends on implicit duplication/majority vote in the form of increased cell size of each bit instead of discrete duplication. Same way as repetition of signal sent over the wire can mean using lower baudrate and holding the signal level for longer time. A bit isn't stored in single atom or electron. A cell storing single bit can be considered a group of smaller cells connected in parallel storing duplicate value. And the majority vote happens automatically in analog form as you read total sum of the charge within memory cell.

Depending on how abstractly you talk about computers (which can be the case when contrasting quantum computing with classical computing), memory can refer not just to RAM but anything holding state and classical computer refer to any computing device including simple logic circuits not your desktop computer. Fundamentally desktop computers are one giant logic circuits.

Also RAID-1 is a thing.

At higher level backups are a thing.

So I would say there enough examples of practically used duplication for the purpose of error resistance in classical computers.

RAID-1 does not do on the fly error detection or correction. When you do a read you read from one of the disks with a copy but don't validate. You can probably initiate an explicit recovery if you suspect there's an error but that's not automatic. RAID is meant to protect against the entire disk failing but you just blindly assume the non-failing disk is completely error free. FWIW no formal RAID level I'm aware of does majority voting. Any error detection/correction is implemented through parity bits with all the problems that parity bits entail unless you use erasure code versions of RAID 6.

The reason things work this way is you'd have 2x read amplification on the bus for error detection and 3x read amplification on the bus for majority-voting error correction & something in the read I/O hot path validating the data reducing latency further. Additionally, RAID-1 is 1:1 mirroring so it can't do error correction automatically at all because it doesn't know which copy is the error-free. At best it can transparently handle errors when the disk refuses to service the request but it cannot handle corrupt data errors that the disk doesn't notice. If you do FDE then you probably would notice corruption at least and be able to reliably correct even with just RAID-1 but I'm not sure if anyone leverages this.

RAID-1 and other backup / duplication strategies are for durability and availability but importantly not for error correction. Error correction for durable storage is typically handled by modern techniques based on erasure codes while memory typically uses Hamming codes because they were the first ones, are cheaper to implement, and match better to RAM needs than Reed Solomon codes. Raptor codes are more recent but patents are owned by Qualcomm; some have expired but there are continuation patents that might cover it.

Yes, and it's worth pointing out these examples because they don't work as quantum memories. Two more: magnetic memory based on magnets which are magnetic because they are build from many tiny (atomic) magnets, all (mostly) in agreement. Optical storage is similar, much like parent's example of a signal being slowly sent over a wire.

So the next question is why doesn't this work for quantum information? And this is a really great question which gets at the heart of quantum versus classical. Classical information is just so fantastically easy to duplicate that normally we don't even notice this, it's just too obvious a fact... until we get to quantum.