In the United States patent system, the inventors and those working closely with the inventors to prepare the patent application have a duty to share information that they have about the relevant prior art. While it may seem strange, these people working to get a patent need to provide the patent examiner with material that the patent examiner may use to reject their patent application. The goal is for relevant information to be considered so that the patent claims that are granted deserve to be granted. Frequently the inventors know of relevant work in that field that may not be known to the patent examiner. So part of the patent application process is to gather up a list of relevant prior art and provide this to the patent examiner. For things other than United States patents and patent applications, one provides a copy of the relevant prior art. This may be a Ph.D. thesis, a patent from a country other than the United States, a journal article, or even a web page or catalog showing an item for sale that shares some characteristics with the new invention. The duty is to share what one has. There is not a duty to spend time searching to find additional information.
The duty to disclose prior art continues until the patent application process ends with either an issued patent or an abandonment of the process.