Certain groups in the population can be ‘over-researched’ – either because they easily defined, or are accessible and convenient for researchers, or because they have characteristics that are distinctive and particularly interesting for researchers.
It’s a good idea, if carrying out research with a particular defined group, in a defined area or setting (e.g. a school, a hospital, a local authority), to try and find out what other research is going on involving the population you hope to study. You might need to look elsewhere if you find that your potential sample is already – or has recently – been involved in someone else’s research.
This is important because over-researching is burdensome for participants – but also because you are likely to get a higher refusal rate (and thus a less representative sample), and possibly poorer quality data.