code: func findDuplicate(blocks []*BasicBlock) *BasicBlock {
  	if len(blocks) < 2 {
  		return nil
  	}
  	if blocks[0] == blocks[1] {
  		return blocks[0]
  	}
  	// Slow path:
  	m := make(map[*BasicBlock]bool)
  	for _, b := range blocks {
  		if m[b] {
  			return b
  		}
  		m[b] = true
  	}
  	return nil
  }
docs: // findDuplicate returns an arbitrary basic block that appeared more
  // than once in blocks, or nil if all were unique.