git-annex/doc/bugs/chunks_not_deduplicated.mdwn

### Please describe the problem.
When data is chunked for storage on a remote, identical chunks may be stored repeatedly with different hashes.

### What steps will reproduce the problem?
```
git annex init .
dd count=16 bs=1024 if=/dev/urandom of=rand1.bin
dd count=15 skip=1 bs=1024 if=rand1.bin of=rand2.bin
git annex add rand1.bin rand2.bin
mkdir data
git annex initremote data type=directory directory=data encryption=none chunk=1024
git annex copy --all --to=data
find data -type f | xargs sha1sum | sort # shows doubly-stored identical chunks
```

### What version of git-annex are you using? On what operating system?
10.20221004-gbf27a02b0 on redhat 7

### Please provide any additional information below.

I imagine an optional upgrade to a new key format for chunks, such that they had their own hashes, and the larger key they were a part of were identified separately (e.g. in data content or metadata). They would then become deduplicated already using existing means.

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

Yesterday I recovered a bricked phone by accessing flash partitions I had backed up with git-annex. I deleted them locally by accident, by git-annex cobbled many of them together from remotes. The ones it didn't find still had their hashes preserved in the repository, and I was able to compare with hashes on the device and reconstructed all but one. The one I didn't reconstruct is regenerateable.

> [[notabug|done]] --[[Joey]]