code: func (c *createCountingFs) ReportDuplicates() string {
  	c.mu.Lock()
  	defer c.mu.Unlock()
  
  	var dupes []string
  
  	for k, v := range c.fileCount {
  		if v > 1 {
  			dupes = append(dupes, fmt.Sprintf("%s (%d)", k, v))
  		}
  	}
  
  	if len(dupes) == 0 {
  		return ""
  	}
  
  	sort.Strings(dupes)
  
  	return strings.Join(dupes, ", ")
  }
docs: // ReportDuplicates reports filenames written more than once.