Duncan v. Gegan was a United States Supreme Court case that addressed the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner who was being held in a federal prison. The case arose when a prisoner, John Duncan, was held in a federal prison in Illinois. Duncan sought a writ of habeas corpus from the state court, claiming that he was being held in violation of the Constitution. The state court granted the writ, and the federal government appealed the decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The Court reasoned that the writ of habeas corpus was a federal remedy, and that the state court did not have the power to interfere with the federal government's authority to imprison individuals. The Court also noted that the writ of habeas corpus was a remedy that could only be used to challenge the legality of a person's detention, and not to challenge the conditions of the detention. The Court's decision in Duncan v. Gegan established that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal prisons. This decision has been cited in numerous subsequent cases, and has been used to support the principle that state courts cannot interfere with the federal government's authority to imprison individuals.

Justice Field delivered the dissenting opinion in Duncan v. Gegan, arguing that the court should not have reversed the decision of the Supreme Court of Illinois. He argued that under Illinois law, a contract was formed when an offer was accepted and consideration given; this had been done in this case and thus a valid contract existed between Duncan and Gegan. Furthermore, he argued that even if there were any ambiguities or uncertainties as to what constituted acceptance of an offer under Illinois law at the time, it was up to state courts to interpret their own laws rather than federal courts doing so on appeal from those decisions. Justice Field concluded by stating his belief that since no error had been committed by either party or by any court below them, they should have affirmed rather than reversed the judgment of lower courts.