The company behind Polygon Zero, the Ethereum scaling solution, said in a tweet on Thursday that Matter Labs, the firm behind the zkSync rollup, copied part of Polygon’s open-source code without providing attribution.

Blockchain projects frequently release their code under open-source software licenses, meaning outside developers are allowed to read, copy, and even (in some cases) contribute to a company’s original codebase. However, community norms – and most open-source licenses – generally require that third-party developers acknowledge when they’ve used code that originated elsewhere.

In a blog post, Polygon alleged that Matter Labs recently released a proving system, called Boojum, that included some source code that had been copy-pasted from Polygon’s own “Plonky2” software library. “This code is included without the original copyrights or clear attribution to the original authors,” Polygon said.

“Copy-pasting source code without attribution and making misleading claims about the original work is against the open source ethos and hurts the ecosystem,” the post read.

Polygon and Matter Labs build competing zero-knowledge rollups – so-called layer 2 blockchains that offload traffic from the “layer 1” Ethereum chain to offer cheaper and quicker transactions for users. This isn’t the first time tensions have flared up between the two projects as they’ve jostled with one another to attract some of the same users and investors.

Read more: War of Words Over zkEVMs Might Portend Long Struggle to Tech Maturity

Other members of Ethereum’s layer 2 ecosystem took the occasion of Polygon’s blog post to share their own disappointment with Matter Labs. Starkware co-founder Uri Kolodny tweeted that it wasn’t “the first time” that a company had copied another team’s code without giving credit, adding, “I’ll bet an ice-cream it won’t be the last time either.” Kolodny didn’t clarify in his tweet whether he was talking about Matter Labs or speaking generally.

Starkware’s Ecosystem Lead, Louis Guthmann, also shared that “These allegations are very serious. Respecting licences and more importantly, honesty and clear attribution is the soul of Open Source.”

CoinDesk reached out to Matter Labs for comment but did not hear back in time for publication.