How to check duplicates in an entire Google Spreadsheet - google-apps-script

Question

I'm trying to accomplish something perhaps to ambitious but perhaps you could give me a hand.
I have a Google Spreadsheet with 4 Sheets:
Master Senpai | Chevez San | Gabbie Sama | Mario Chan
"Master Senpai" is a sheet with Consolidated data meaning everything in the other 3 sheets is within "Master Senpai"
These spreadsheet is currently being use by 3 coworkers (as you might guess Chevez Gabbie and Mario) what I need is for each of them to have the ability to confirm or check if any of the info they are inputting is duplicated.
All the Sheets have the same headers from A to L (12 Columns total) I want to check the duplicates for Column J.
So for Example if Mario Want to know if the info he just inputted Gabbie or Chevez has already put it I want that cell to get Formated with a red background.
Basically read the info from the last 3 sheets (Ignore the "Master Senpai" sheet) and check for duplicates if any found color them with a red background.
I've been reading about it and found something similar to what I'm trying to do.
/**
* Finds duplicate rows in the active sheet and colors them red,
but only pays attention to the indicated columns.
*/
function findDuplicates() {
// List the columns you want to check by number (A = 1)
var CHECK_COLUMNS = [2,3,5,6];
// Get the active sheet and info about it
var sourceSheet = SpreadsheetApp.getActiveSheet();
var numRows = sourceSheet.getLastRow();
var numCols = sourceSheet.getLastColumn();
// Create the temporary working sheet
var ss = SpreadsheetApp.getActiveSpreadsheet();
var newSheet = ss.insertSheet("FindDupes");
// Copy the desired rows to the FindDupes sheet
for (var i = 0; i < CHECK_COLUMNS.length; i++) {
var sourceRange = sourceSheet.getRange(1,CHECK_COLUMNS[i],numRows);
var nextCol = newSheet.getLastColumn() + 1;
sourceRange.copyTo(newSheet.getRange(1,nextCol,numRows));
}
// Find duplicates in the FindDupes sheet and color them in the main sheet
var dupes = false;
var data = newSheet.getDataRange().getValues();
for (i = 1; i < data.length - 1; i++) {
for (j = i+1; j < data.length; j++) {
if (data[i].join() == data[j].join()) {
dupes = true;
sourceSheet.getRange(i+1,1,1,numCols).setBackground("red");
sourceSheet.getRange(j+1,1,1,numCols).setBackground("red");
}
}
}
// Remove the FindDupes temporary sheet
ss.deleteSheet(newSheet);
// Alert the user with the results
if (dupes) {
Browser.msgBox("Possible duplicate(s) found and colored red.");
} else {
Browser.msgBox("No duplicates found.");
}
};
/**
* Adds a custom menu to the active spreadsheet
*/
function onOpen() {
var sheet = SpreadsheetApp.getActiveSpreadsheet();
var entries = [{
name : "Find Duplicates",
functionName : "findDuplicates"
}];
sheet.addMenu("My Scripts", entries);
fillFormulas();
};
However this only looks for duplicates in the same sheet what I'm trying to accomplish is to read and compare the info from the last 3 sheets.