The general policy of the XMM-Newton Observatory is to avoid repeating the same observation, i.e. to avoid duplications.
In general, a duplication is determined by consideration of the target coordinates and of the main observing parameters (especially the instrument(s) and the observing modes). A proposed observation duplicates another one if the expected science data are essentially the same or of lower quality (e.g. lower exposure time). It is, however, allowed to observe the same target with the same instrument configuration several times for variability studies. On the other hand, observations of hard X-ray sources with EPIC using different filters only (which mainly affects the soft energy response) may be classified as a duplication.
The responsibility for defining and resolving cases of duplication rests with the OTAC in consultation with the Project Scientist as needed. The OTAC can allow duplications between a proposed observation and an observation of a previous cycle. These should be restricted to proposals which provide convincing evidence that additional data are of scientific relevance and that the scientific case cannot be fulfilled with the existing data.
All observations which are not considered as being successfully observed, as well as the calibration observations already foreseen at the time of the announcement are made public for each Announcement of Opportunity. Proposals in response to this call are not permitted to duplicate these observations. Proposers are requested to check for duplications of their preferred targets against already performed or planned observations, using the XMM-Newton ``Target Search Tool'' or ``Observation Lokator'' web tool (http://www.cosmos.esa.int/web/xmm-newton/target-search-tool). This tool includes all types of XMM-Newton observations like open, TOO and routine calibration.