package main

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

func main() {
	reader := bufio.NewReader(os.Stdin)
	writer := bufio.NewWriter(os.Stdout)
	defer writer.Flush()

	var t int
	fmt.Fscan(reader, &t)

	for tc := 0; tc < t; tc++ {
		var n int
		fmt.Fscan(reader, &n)

		f := make([]int, n+1)
		for i := 0; i < n; i++ {
			var x int
			fmt.Fscan(reader, &x)
			if x <= n {
				f[x]++
			}
		}

		c1_found := false
		ans := -1
		for i := 0; i <= n; i++ {
			if f[i] == 0 {
				ans = i
				break
			}
			if f[i] == 1 {
				if !c1_found {
					c1_found = true
				} else {
					ans = i
					break
				}
			}
		}
		fmt.Fprintln(writer, ans)
	}
}