In college we referred to this same idea (among my peers who were going on through graduate studies, particularly PhDs) as "academic incest", where you aren't sufficiently exposed to a diverse enough set of ideas and end up too narrowly focused or clouded in your judgment.
Ways to overcome it include: moving (in academics, accepting a fellowship at other institutions; as a professional, change jobs); observation (keeping abreast of the latest trends outside your immediate environment and field); community participation (attending conferences, symposia, and similar things; or even forums like this).
But ultimately it requires a recognition that your views, knowledge, and beliefs are, necessarily, constrained by your experiences and that you sometimes have to seek out novel experiences or the experiences of others to change yourself. If you lack the drive for either, or too great a confidence in your own present state, then you're going to be stuck.
Ways to overcome it include: moving (in academics, accepting a fellowship at other institutions; as a professional, change jobs); observation (keeping abreast of the latest trends outside your immediate environment and field); community participation (attending conferences, symposia, and similar things; or even forums like this).
But ultimately it requires a recognition that your views, knowledge, and beliefs are, necessarily, constrained by your experiences and that you sometimes have to seek out novel experiences or the experiences of others to change yourself. If you lack the drive for either, or too great a confidence in your own present state, then you're going to be stuck.