all: functions deduplication

[tarasmadan]

Is your feature request related to a problem? Please describe.
We have 4 functions func unique([]string) []string.
How to detect more duplicates?

Describe the solution you'd like
Maybe our linter requires tuning.

Additional context
#6353 (comment)

[dvyukov]

We already have dupl checkers, and it's workting:
https://github.com/google/syzkaller/blob/master/.golangci.yml#L74-L75

It has threshold on the number of tokens. If you want to lower it, it will require more cleanup. But my memories at low values it starts producing what I would treat as false positives.

[dvyukov]

Yup, still working:

bin/golangci-lint run ./...
pkg/codesearch/codesearch.go:104: 104-111 lines are duplicate of `pkg/codesearch/codesearch.go:113-120` (dupl)
Error: 104-111 lines are duplicate of `pkg/codesearch/codesearch.go:113-120` (dupl)
pkg/codesearch/codesearch.go:113: 113-120 lines are duplicate of `pkg/codesearch/codesearch.go:104-111` (dupl)
Error: 113-120 lines are duplicate of `pkg/codesearch/codesearch.go:104-111` (dupl)
2 issues:
* dupl: 2
Error: Process completed with exit code 2.

https://github.com/google/syzkaller/actions/runs/19426660550/job/55575712826?pr=6458