Using golang easier to generate only letters by random.

GenRndLetter.go

package main

import (
	"crypto/rand"
	"fmt"
	"io"
)

func newId(size int) string {
	k := make([]byte, size)
	if _, err := io.ReadFull(rand.Reader, k); err != nil {
		return ""
	} else {
		for i, v := range k {
			x := 65 + v%26
			k[i] = x
		}
		return string(k)
	}
}

func main() {
	size := 10000000
	var slot = make(map[string]struct{}, size)

	for i := 0; i < size; i++ {
		k := newId(10)
		if _, ok := slot[k]; !ok {
			slot[k] = struct{}{}
			//			fmt.Println(k)
		} else {
			fmt.Println("Same", k, i)
			panic(k)
		}
	}
}