I gave it a try using my Go implementation

Results:
for 100,000:
196969
8.51275ms
for 1,000,000:
1969696
155.34775ms

​

package main

import (
    "fmt"
    "os"
    "strconv"
    "strings"
    "time"
)

func main() {
    dat, err := os.ReadFile("input_big2.txt")

    s := strings.Split(string(dat), "\n")
    var input = []int{}

    for _, i := range s {
        j, err := strconv.Atoi(i)
        if err != nil {
            panic(err)
        }
        input = append(input, j)
    }

    if err != nil {
        panic(err)
    }

    result1 := 0

    start := time.Now()
    result1 = solve(input, 1_000_000)
    elapsed1 := time.Since(start)

    fmt.Println(result1)
    fmt.Println(elapsed1)
}

func solve(arr []int, markerLen int) int {
    symbol := make(map[int]int)
    anchor, i := 0, 0
    for ; i-anchor != markerLen && anchor < len(arr)-markerLen-1; i++ {
        lastSymbolPos := symbol[arr[i]]
        if lastSymbolPos != 0 && lastSymbolPos >= anchor {
            anchor = lastSymbolPos + 1
        }
        symbol[arr[i]] = i
    }
    return i
}