code: func (m Matrix) ContainsSameLabelset() bool {
  	l := make(map[uint64]struct{}, len(m))
  	for _, ss := range m {
  		hash := ss.Metric.Hash()
  		if _, ok := l[hash]; ok {
  			return true
  		}
  		l[hash] = struct{}{}
  	}
  	return false
  }
docs: // ContainsSameLabelset checks if a matrix has samples with the same labelset
  // Such a behavior is semantically undefined
  // https://github.com/prometheus/prometheus/issues/4562