Cannot Read Duplicate Values for Shadowed Keys #284

Describe the bug
Duplicate values for shadowed keys are not preserved in either ValueWithShadows() or just WriteTo().

To Reproduce

package main

import (
	"os"

	"gopkg.in/ini.v1"
)

const f = `[test]
Key1=Key1Value1
Key1=Key1Value2
Key1=Key1Value1
`

func main() {
	cfg, _ := ini.ShadowLoad([]byte(f))
	cfg.WriteTo(os.Stdout)
}

produces

[test]
Key1 = Key1Value1
Key1 = Key1Value2

Expected behavior
Reading in the file should not destroy the duplicate line. WriteTo() should reproduce the source file:

[test]
Key1 = Key1Value1
Key1 = Key1Value2
Key1 = Key1Value1

Additional context
While similarly named, this is not a duplicate of #283 which is a case of needing ValueWithShadows().

I am working with a save file for a game that is stored in an INI format. Extra equipment may be stored in the cargo with lines such as

cargo                = 2322110156, 1, , , 0
cargo                = 2859002568, 1, , , 0
cargo                = 2859002568, 1, , , 0

where the last two duplicate lines indicate additional quantity. Currently, go-ini is mangling the duplicate lines resulting in the loss of equipment.