How to ensure a research idea has not been explored previously
This is of course always hard to protect oneself from. However, I would say the best way to ensure that your research proposal hasn't been investigated before is to get in contact with an expert in the field and ask him or her this.

Of course, you should investigate the topic thoroughly before approaching the expert. One tip to do this is by using a citation index. This enables you to track how a certain paper has been cited, meaning you can follow the trail and more effectevly find out what has been done. For example, if you have located an old seminal paper in your field, finding out which papers have referenced this one should give you a fair chanse of finding what you're looking for.