Duplo

Overview

Duplo is a free, self-hosted duplicate-code detector for repository scans, best suited to developers who want a focused way to identify repeated blocks and feed results into review or CI workflows. Its language-aware filtering and machine-readable reports make it practical for that job, but the default 20-line threshold and GPL terms deserve attention before adopting it.

It can scan arbitrary text as well as source code, including repetitions within one file, and accepts files through standard input or a separate file list. That flexibility suits scripted workflows; the text-based clone detection remains narrowly focused on duplication rather than broader code quality.

Key features

Language-aware scanning

Built-in support covers C/C++, Java, C#, VB, GCC assembly, Ada, Erlang, TypeScript, and Swift. For supported languages, Duplo can strip comments, preprocessor directives, and other syntax before comparing blocks. This helps teams focus on code structure instead of matching incidental syntax, while arbitrary text scanning extends its use beyond source files.

Reports and CI

Duplo produces text, JSON, and XML reports. XML can be used with CI tools such as GitHub Actions and TeamCity, and the maker links a GitHub Action that can fail a build when duplicates are found. The action offers configurable minimum duplicate lines, minimum line length, and maximum files to report, which gives teams control over noise and report size.

The quick-start example requires at least 20 lines for a duplicate, though command-line options can change that threshold. A high minimum can miss shorter repetition; lowering it may bring more matches to assess. The README reports that processing Quake2’s 266 files and 102,740 lines took 9 seconds on modern hardware in 2025, and says multithreading can improve performance almost linearly. That gives an indication of intended scale, not a guarantee for every repository or machine.

Pricing

Duplo is free software: the Duplo plan costs 0.00 USD per free and includes prebuilt binaries for Alpine Linux, macOS, and Windows. There is no paid tier or trial to weigh. Users can also build it from source. The trade-off is licensing: Duplo is distributed under GNU GPL version 2 or, at the user’s option, any later version, so teams should check that its terms fit their use.

Platforms

Duplo supports Linux, macOS, and Windows, with prebuilt binaries available for Alpine Linux, macOS, and Windows. It is self-hosted, making it appropriate for teams that want to run repository scans in their own environment rather than rely on a hosted service.

Who it's for

Duplo fits developers and teams that need a focused duplicate-block scan across large codebases, especially when they want text, JSON, or XML output or a build-failing GitHub Action. Its broad language list and arbitrary-text support are useful when a repository contains more than conventional source files. It is less suitable for teams seeking a general code-quality platform or unable to accommodate GPL licensing and the stated warranty disclaimer.

Pros and cons

• Useful language and text coverage: built-in syntax filtering for nine language groups, plus arbitrary text scans, gives it reach beyond one language or source format.
• Workflow-friendly output: JSON and XML reports, standard-input and file-list inputs, and a linked GitHub Action support scripted and CI use.
• Threshold needs tuning: the quick-start’s 20-line minimum may not suit teams looking for shorter duplicate blocks.
• Warranty disclaimer: the license section disclaims warranty, including implied warranties of merchantability and fitness for a particular purpose.

Alternatives

Browse Code Clone Detection Tools for more options. Choose PMD if a BSD-style licensed downloadable static code analyzer better matches your needs. NiCad is another free option, though it requires OpenTxl 11 or later. Simian Similarity Analyzer is free and Apache License 2.0-licensed. Consider CloneWorks if its acknowledgement and free redistribution and derivative-work conditions suit your use. dcd is a free option for Linux and macOS under AGPL-3.0; it also offers priority direct email support from the developer, with custom pricing. jscpd, copydetect, and Hound are other alternatives to compare.

Verdict

Choose Duplo if you want a free, self-hosted duplicate-code scanner with language-aware filtering and reports that can feed CI. Its main advantage is the combination of broad text and language coverage with configurable build checks; look elsewhere if its line threshold, GPL terms, or warranty disclaimer do not fit your team.