Current implementation of Collectors.averagingDouble calls the supplied mapper twice per accumulated element:

(a, t) -> { sumWithCompensation(a, mapper.applyAsDouble(t)); a[2]++; a[3]+= mapper.applyAsDouble(t);},

Similarly Collectors.summingDouble:

(a, t) -> { sumWithCompensation(a, mapper.applyAsDouble(t));
           a[2] += mapper.applyAsDouble(t);},

As mapper is a user-supplied function, it's not guaranteed to be a simple getter. Actually it could be computationally intensive. For example, the following code could be used to find average distance of the set of points from the given central point:

  static class Point {
    int x, y;

    Point(int x, int y) { this.x = x;this.y = y; }

    double distance(Point p) {
      return Math.hypot(x - p.x, y - p.y);
    }
  }

  public double averageDistance(Point center, List<Point> list) {
    return list.stream().collect(Collectors.averagingDouble(center::distance));
  }

Calling the mapper twice is absolutely unnecessary and may affect the performance when mapper is non-trivial.