Note: This post was written over 3 months ago. Generative AI is a rapidly changing field. While we try to provide durable advice based on consistent principles, specific recommendations or instructions may have changed based on new facts since this was written.

Canonical reference

This post is part of the Doppelgänger Hallucination failure-mode entry. The canonical reference page is the authoritative definition and citation source; this post is the chronological recap of the test series.

Back in October 2025, I coined the phrase Doppelgänger Hallucination after a hunch about LLM hallucinations led me to find that both Google AI Overviews and Perplexity "confirmed" the existence of nonexistent legal cases.

Doppelgänger Hallucination: when one LLM asked about a second LLM's ungrounded hallucination without additional context or prompting provides a false confirmation of the hallucination; this may also include embellishment with additional details. Not to be confused with both LLMs retrieving answers from the same bad source of data.

Google AI Overview hallucinating details about the fictitious case Weber v. City of Cape Girardeau

Google AI Overview confidently describing the fictitious case Weber v. City of Cape Girardeau, 447 S.W.3d 885 (Mo. App. 2014) — a case that does not exist, cited in Kruse v. Karlen.

I’ve gone through:

Tested 2 fake cases from Grant v. City of Long Beach
Tested 3 fake cases from United States v. Cohen
Tested 22 fake cases from Kruse v. Karlen
Tested 4 fake cases from Smith v. Farwell
Tested 1 fake case from Park v. Kim
Tested 1 fake case from In re Turner

AI Gone Wrong in the Midwest

I have gotten onboarded with my software vendor to provide my CLEs available on-demand and am working on accreditation for several states and I will announce those as they are approved.

I discuss the Doppelgänger Hallucination Test in the context of In re Turner for “AI Gone Wrong in the Midwest,”.