# Adds a second library for a job an installed one already does (dayjs beside moment)

*Code that already exists*: one of the ways AI coding agents bloat software, from lessloc's [pattern catalog](https://lessloc.org/patterns/index.md). What lessloc does: [`new-dependency`](https://lessloc.org/guide/reference.md#tier-2) names the library that is already installed.

## What makes a second library for a job an installed one already does a bad practice?

The standards that call it a bad practice: One obvious way to do it ([PEP 20](https://peps.python.org/pep-0020/)); OWASP Top 10 A06:2021 and A03:2025 Software Supply Chain Failures.

## What does lessloc do about a second library for a job an installed one already does?

[`new-dependency`](https://lessloc.org/guide/reference.md#tier-2) names the library that is already installed.

- `new-dependency` (tier 2): A new dependency, when there is no `approvedDependencies` list. It says when an installed one already does the job, or when the lockfile does not pin it.

## Related patterns

The other patterns in [Code that already exists](https://lessloc.org/patterns/index.md#code-that-already-exists):

- [Reimplementing a helper the project already has](https://lessloc.org/patterns/reuse-helper.md): lessloc gives every session the rule: take what is already in the codebase before new code.
- [Copying a rule another function owns](https://lessloc.org/patterns/copied-rule.md): lessloc catches it as `repeated-literal`.
- [Keeping old and new versions side by side](https://lessloc.org/patterns/side-by-side.md): lessloc gives every session the rule: one implementation per rule.
- [Rebuilding what the standard library or framework has](https://lessloc.org/patterns/rebuilt-library.md): lessloc gives every session a rule against it and catches it as `simpler` and `reinvented`.
- [Copy-pasted test cases instead of one parametrized test](https://lessloc.org/patterns/copied-tests.md): lessloc catches it as `copied-tests`.
- [The same data shape in several layers (DTO, model, entity)](https://lessloc.org/patterns/duplicate-shapes.md): `copy` catches identical type bodies.
- [The same CSS declarations under another selector](https://lessloc.org/patterns/css-copy.md): lessloc catches it as `style-copy`.