Check duplicates between all sheets in google sheets - google-apps-script

Question

What I want to do is check for duplicates between all google sheets and if there are any - mark them in a color. For now, I left the headers to my columns just to see if it marks them red. I found one script that everyone said that worked - however, I got really random results with it. Could you possibly help with it?
The script:
Array.prototype.countItem = function (item) {
var counts = {};
for (var i = 0; i < this.length; i++) {
var num = this[i];
counts[num] = counts[num] ? counts[num] + 1 : 1;
}
return counts[item] || 0;
}
function findDuplicatesOnAllSheets() {
var nondupes = [];
SpreadsheetApp.getActive()
.getSheets()
.forEach(function (s) {
s.getRange('A1:E500')
.getValues()
.reduce(function (a, b) {
return a.concat(b);
})
.forEach(function (x, i, v) {
if (x && nondupes.countItem(x) == 0) {
nondupes.push(x)
} else if (x && nondupes.countItem(x) >= 1) {
s.getRange(i + 1, 3)
.setBackground('red');
}
});
});
}
This is the sheet I tested it on and you can see starting from sheet 3 that the results were random. The file should only have the headers as duplicates.
https://docs.google.com/spreadsheets/d/1naYaxR0f_wGsRP4-nBRZfAJ9ZqxpyA1uqQ9Uz5xvwNs/edit#gid=1044860260