You mention that the field is computer science, and one is a workshop paper and the other is a conference paper.
It is not uncommon that a paper is first submitted and presented at a workshop that is attached to a conference, often in a preliminary state, and a more complete version is later submitted and published at a future conference. This second submission is allowed by many conferences when the workshop it was first presented in had no formal proceedings, and so the paper was not considered formally published there. There is often no requirement for the second submission to have new content compared to the first. The paper may also still appear on the workshop's website in such cases, but no copyright transfer takes place.
It is possible that the situation in your case is something similar.
Examples of such policies:
Issuing the paper as a technical report, posting the paper on a web site, or presenting the paper at a workshop or conference that does not publish formally reviewed proceedings does not disqualify it from appearing in an ACM publication. Workshops and conferences are encouraged to indicate in their calls for papers whether or not they will publish formally reviewed proceedings so that authors can determine whether or not submission will jeopardize ACM publication.
Submission is permitted for papers presented or to be presented at conferences or workshops without proceedings (e.g., ICML or NeurIPS workshops), or with only abstracts published.