Users are responsible for checking for duplicate or potentially duplicate observations, and explaining any such observations in their proposals.
See also: JWST Duplication policy
As part of maximizing the science return of JWST, unnecessary duplication of observations need to be avoided. Proposed observations that duplicate existing or planned observations must be scientifically justified by the proposer. The JWST Duplication Policy defines what constitutes a duplication. In short, a proposed observation is a duplication if some combination of the following are true: a) same source or field; b) same or similar instrument; c) same mode or template; d) similar (within a defined factor) exposure time or Signal-to-noise ratio; and e) for spectral modes, similar spectral resolution and/or significantly overlapping spectral coverage.
Duplication checking prior to submission is the user's responsibility. Any duplications or potential duplications (for programs where the details are not known up front) must be discussed and/or justified in the text of the proposal. Failure to do so may result in the rejection of the proposed observation even after proposal acceptance. For accepted proposals, STScI will perform a detailed duplication check to catch and remove any duplications within the current pool of accepted proposals.
Because of the timing of final GTO and ERS inputs, the full listing of accepted observations is not available as of the release of the Call for Proposals for for Cycle 1. The full listing and instructions on its use will be provided at least 8 weeks prior to the proposal deadline. This article will be updated at that time.
To avoid unintentional duplications, proposers will be required to check their proposed observations against those already approved (planned or actually observed in future cycles). When valid JWST observations have been obtained and archived, users will be able to go to the Mikulski Archive for Space Telescopes (MAST) and use the available tools to ensure their proposed targets are not duplicating existing or planned JWST observations. For Cycle 1 General Observers, there are of course no existing observations, but Guaranteed Time Observer (GTOs) and Early Release Science (ERS) proposals have been accepted prior to Cycle 1. Note that targets will not be protected, but rather observations with a particular instrument and instrument mode of targets are protected against unjustified duplications. Many users will want to do this duplication checking step prior to going to the effort of entering and validating an APT proposal, to avoid the potential of only finding out after the fact that duplications have been proposed.
Once a proposal is accepted, one of this first steps in proposal processing is a duplication check performed by STScI personnel. The STScI software not only checks for duplications against previous and planned observations, but also checks within the ensemble of accepted programs for a given cycle to catch any potential duplications. STScI personnel will contact affected PIs to resolve any such duplications if they occur. Any duplications found against previous or planned observations that were not revealed by the PI and explicitly approved by the TAC and STScI director may be disallowed and the program's resource allocation decreased accordingly.