Adding insulation to basement that already has vapor barrier
Insulation is always installed as such:

Warm side | Vapor/ Moisture barrier | Insulation | Cold side

You ONLY want a moisture barrier between the warm side and the insulation, because warm air holds onto more moisture than cold air, and when warm air meets cold (air, wall, whatever), the air has to let go of that moisture as it cools, forming condensation.  The moisture barrier prevents that moist air from getting into the insulation where cold and warm meet.

If you put insulation on the wrong side of your barrier, the moisture will form in the insulation, and as Shirlock said, if you create TWO layers of barrier, you create a potential moisture trap.