Duplo

Duplo is a free, self-hosted tool for finding repeated code blocks in large repositories, best suited to developers who want a focused duplicate scan in a local or CI workflow. Its language-aware filtering and multiple report formats are useful strengths, but the quick-start threshold and GPL license terms deserve consideration before adoption.

Overview

Duplo uses text-based clone detection to scan repositories, including arbitrary text files and repetitions within a single file. Inputs can come through standard input or a separate file list, giving scripted workflows a straightforward way to define what gets scanned. It focuses on duplication rather than broader code-quality analysis, so it is a poor fit for teams seeking a general static analyzer.

For supported languages, it can remove comments, preprocessor directives, and other syntax before checking for duplicate blocks. That filtering can help distinguish repeated code from incidental textual matches. The quick-start example requires at least 20 lines for a match, though command-line options can change the threshold; teams looking for shorter clones should tune it, while a lower threshold may bring more noise.

Key features

Language-aware scans and reports

Built-in support covers C/C++, Java, C#, VB, GCC assembly, Ada, Erlang, TypeScript, and Swift, with arbitrary text files as an additional target. Text, JSON, and XML output provide options for direct review or downstream processing. XML is described for use with CI tools such as GitHub Actions and TeamCity.

Build integration and performance

A linked GitHub Action can fail a build when duplicate blocks are found. Its configurable minimum duplicate-line count, minimum line length, and maximum files to report help teams balance detection against noisy or oversized reports. The README reports a scan of Quake2's 266 files and 102,740 lines taking 9 seconds on modern hardware in 2025, and says multithreading can improve performance almost linearly. That example is encouraging, but it is not a performance guarantee for other repositories or machines.

Pre-built binaries are available for Alpine Linux, macOS, and Windows, and Duplo can also be built from source. The README directs feedback, feature requests, and bug reports to an issue tracker. The project is free software under GNU GPL version 2 or, at the user's option, any later version, and is provided without warranty, including implied warranties of merchantability or fitness for a particular purpose.

Pricing

Duplo: free. The plan is free software under GNU GPL v2 or later, with pre-built binaries for Alpine Linux, macOS, and Windows. There is no paid tier or free trial to weigh; the material trade-off is the license. Teams should assess whether GPL terms suit their intended use, and the warranty disclaimer makes it unsuitable for anyone requiring a software warranty.

Platforms

Duplo supports Linux, macOS, and Windows. It is self-hosted, with pre-built binaries for Alpine Linux, macOS, and Windows and a source-build option. That suits teams able to manage a command-line tool themselves, rather than buyers seeking a hosted service.

Who it's for

Duplo is a sensible choice for developers who need repository-level duplicate detection across its supported languages or arbitrary text, especially when structured JSON or XML output and a build-failing GitHub Action fit an existing workflow. It is less suitable for teams wanting broad code-quality checks, short-clone detection without threshold tuning, hosted operation, or warranty-backed software.

Pros and cons

• Pros: Language-aware filtering can strip comments and other syntax, helping focus matches on code rather than incidental text.
• Pros: Text, JSON, and XML reports, plus a configurable GitHub Action, support both manual review and CI use.
• Pros: Free software and binaries for three major desktop/server platforms make it accessible to teams comfortable with self-hosting.
• Cons: The quick-start's 20-line minimum can miss shorter duplicate blocks unless the threshold is adjusted.
• Cons: GPL v2-or-later licensing and the no-warranty disclaimer may not suit every organizational or risk requirement.
• Cons: It is a focused clone detector, not a general code-quality analyzer.

Alternatives

Consider PMD when a downloadable static code analyzer under a BSD-style license better suits your needs. CloneWorks is another free option, but its use requires acknowledgement and its redistribution and derivative terms require that they remain free of charge. Choose dcd if an AGPL-3.0 open-source license fits; it also offers paid priority direct-email support billed a year's worth. jscpd is another free option for Linux, macOS, Windows, API, or self-hosted use. NiCad may suit teams preferring MIT-licensed software, provided they can use OpenTxl 11 or later. Simian Similarity Analyzer is a free Apache License 2.0 open-source alternative. Algo VPN is another free option. copydetect is a free alternative with API and self-hosted platform options. Browse the Code Clone Detection Tools list to compare more options.

Verdict

Choose Duplo if you need a free, self-hosted duplicate detector with language-aware filtering and CI-friendly reports, and can accept GPL terms and tune the minimum match size to your codebase. Look elsewhere if you need broader static analysis, a hosted service, or software backed by a warranty.