'The Go Programming Language', Exercise 1.4

a_main.go

// Exercise 1.4 from 'The Go Programming Language', by A Donovan and
// B Kernigham
//
// Zach Young
// 27 May, 2019
//
//Dup2 prints the count and text of lines that appear more than once
//in the input.  It reads from stdin or from a list of named files.

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

func main() {
	counts := make(map[string]map[string]int)
	files := os.Args[1:]
	if len(files) == 0 {
		countLines(os.Stdin, counts, "Stdin")
	} else {
		for _, arg := range files {
			f, err := os.Open(arg)
			if err != nil {
				fmt.Fprintf(os.Stderr, "dup: %v\n", err)
				continue
			}
			countLines(f, counts, arg)
			f.Close()
		}
	}
	for line, nameCount := range counts {
		n := 0
		names, sep := "", ""
		for name, count := range nameCount {
			names += sep + name + "[" + strconv.Itoa(count) + "]"
			sep = " "
			n += count
		}
		if n > 1 {
			fmt.Printf("%d\t%s\t(%s)\n", n, line, names)
		}
	}
}

func countLines(f *os.File, counts map[string]map[string]int, name string) {
	input := bufio.NewScanner(f)
	for input.Scan() {
		line := input.Text()
		if counts[line] == nil {
			counts[line] = make(map[string]int)
		}
		counts[line][name]++
	}
	// NOTE: ignoring potential errors from input.Err()
}

dup1.txt

aaa
aaa
aaa
aaa
bbb
bbb
zzz

dup2.txt

xxx
xxx
xxx
bbb
bbb
bbb
bbb
bbb
zzz
zzz
zzz

dup3.txt

zzz
zzz
zzz
zzz
xxx
xxx
xxx
xxx
xxx
yyy
yyy
yyy
yyy
yyy

run.sh

$ ./main dup*.txt | sort
  4	aaa	(dup1.txt[4])
  5	yyy	(dup3.txt[5])
  7	bbb	(dup1.txt[2] dup2.txt[5])
  8	xxx	(dup2.txt[3] dup3.txt[5])
  8	zzz	(dup1.txt[1] dup2.txt[3] dup3.txt[4])

$ cat *.txt | ./main
  8	zzz	(Stdin[8])
  8	xxx	(Stdin[8])
  5	yyy	(Stdin[5])
  4	aaa	(Stdin[4])
  7	bbb	(Stdin[7])

Hey man! I found your answer, pretty neat!
I made some changes of my own if you'd like to check: https://gist.github.com/ironknight78/082de37afffd13ad6df62a30f90bb262

Hi, @ironknight78! Thanks for the message, I'm glad it caught your attention :)