Code to generate powerset in Golang gives wrong result

Next code in Golang to generate powerset produces wrong result on input {"A", "B", "C", "D", "E"}. I see [A B C E E] as the last generated set.

<!-- language: go -->

package main

import (
    "fmt"
)

func main() {
	for _, s := range PowerSet([]string{"A", "B", "C", "D", "E"}) {
		fmt.Println(s)	
	}	
}

func PowerSet(set []string) [][]string {
	var powerSet [][]string
	powerSet = append(powerSet, make([]string, 0))
	for _, element := range set {
		var moreSets [][]string
		for _, existingSet := range powerSet {
			newSet := append(existingSet, element)
			moreSets = append(moreSets, newSet)
		}
		powerSet = append(powerSet, moreSets...)
	}
	return powerSet
}

How to fix it? How to write it idiomatically in Go?

The problem with your program is not the algorithm itself but this line:

newSet := append(existingSet, element)

You should not append and assign to a different variable.

As the documentation states (emphasis mine), "The append built-in function appends elements to the end of a slice. If it has sufficient capacity, the destination is resliced to accommodate the new elements. If it does not, a new underlying array will be allocated.".

So, there might be cases where newSet := append(existingSet, element) will actually modify existingSet itself, which would break your logic.

If you change that to instead create a new array and append to that one, it works as you expect it.

newSet := make([]string, 0)
newSet = append(newSet, existingSet...) 
newSet = append(newSet, element)

For instance, you can use algorithm like this one: https://stackoverflow.com/a/2779467/3805062.

func PowerSet(original []string) [][]string {
    powerSetSize := int(math.Pow(2, float64(len(original))))
    result := make([][]string, 0, powerSetSize)

    var index int
    for index < powerSetSize {
	    var subSet []string

	    for j, elem := range original {
		    if index& (1 << uint(j)) > 0 {
			    subSet = append(subSet, elem)
		    }
	    }
	    result = append(result, subSet)
	    index++
    }
    return result
}

Elaborating on @eugenioy's answer.
Look at this thread. Here is a working example : https://play.golang.org/p/dzoTk1kimf

func copy_and_append_string(slice []string, elem string) []string {
	// wrong: return append(slice, elem)
	return append(append([]string(nil), slice...), elem)
}

func PowerSet(s []string) [][]string {
	if s == nil {
		return nil
	}
	r := [][]string{[]string{}}
	for _, es := range s {
		var u [][]string
		for _, er := range r {
			u = append(u, copy_and_append_string(er, es))
		}
		r = append(r, u...)
	}
	return r
}