Karl Friedrich Gauss made many discoveries that he did not publish and that remained unknown until later mathematicians (re)discovered them. When Gauss's personal notebooks were later examined, it turned out that he had made the same discoveries decades earlier.
For example, in Visual Complex Analysis Tristan Needham discusses how Hamilton and Rodrigues were apparently the first to discover quaternions in 1843 and 1840, respectively. Only later did it turn out that Gauss had preceded them by more than 20 years. Needham writes:
"Hamilton and Rodrigues are just two examples of hapless mathematicians who would have been dismayed to examine the unpublished notebooks of the great Karl Friedrich Gauss. There, like just another log entry in the chronicle of his private mathematical voyages, Gauss recorded his discovery of the quaternion rule in 1819."
What other significant discoveries did Gauss make but not publish that were rediscovered by and attributed to later mathematicians?