How to Highlight Duplicates in Google Sheets Guide

The Hidden Cost Of Duplicate Data

Picture this: it is 6 p.m., you are skimming a lead list in Google Sheets before a big launch. Somewhere in those 8,000 rows are duplicate emails. Send to them twice and you look spammy; miss them and you under-report conversion. The work is simple, but the stakes are real.

The good news: highlighting duplicates in Google Sheets is straightforward. The better news: once you know the basics, you can hand the whole workflow to an AI agent like Simular so it runs automatically every time new data arrives.

1. Manual Highlighting With Conditional Formatting

This is your baseline. You will use conditional formatting plus the COUNTIF / COUNTIFS functions.

A. Highlight Duplicates In One Column

1. Select the column you want to scan, e.g., column B with email addresses.
2. Go to Format → Conditional formatting.
3. In the right sidebar, confirm the Apply to range (for example B2:B1000).
4. Under Format cells if, choose Custom formula is.
5. Enter: =COUNTIF($B$2:$B,$B2)>1
6. Pick a bold fill color (e.g., red) so duplicates stand out.
7. Click Done.

Now any value that appears more than once in that column is highlighted. No sorting, no manual scanning.

Pros

• Fast to set up.
• Great for one-off checks or small sheets.

Cons

• You must remember to reapply or adjust ranges as data grows.
• Easy to misconfigure ranges or formulas when you are tired.

B. Highlight Duplicates Across Multiple Columns

Sometimes a single column is not enough. Maybe a customer name can repeat, but the combination of Name + Email should be unique.

1. Select the full range, e.g., A2:C2000.
2. Go to Format → Conditional formatting.
3. Under Custom formula is, use: =COUNTIFS($A$2:$A,$A2,$B$2:$B,$B2)>1
4. Choose a highlight style and click Done.

This checks whether the combination of values (like Product Name + Supplier) appears more than once.

Pros

• Captures real-world uniqueness rules (client + campaign, SKU + region, etc.).

Cons

• Formulas become harder to maintain as you add more columns.
• Still a manual, one-sheet-at-a-time process.

2. Semi-Automation With Templates And Add-ons

Before we bring in a full AI agent, you can speed things up with reusable patterns.

A. Turn Your Rules Into A Template Sheet

1. Create a new Google Sheets file called Data-Cleaning-Template.
2. Build your conditional formatting rules once (single-column and multi-column).
3. When you have a new dataset, copy it into this template instead of starting from scratch.

This is like building your own small product: the rules live in one place, and you re-use them over and over.

Pros

• Consistent rules across projects.
• Less rework for your team.

Cons

• Still requires manual copy-paste.
• Easy for teammates to accidentally break rules.

B. Use A Duplicate-Detection Add-on

There are third-party add-ons that scan for duplicates and highlight them via a UI. You configure the columns and thresholds, hit Run, and the add-on does the rest.

Pros

• Less time wrestling with formulas.

Cons

• Another tool to manage and pay for.
• Still lives inside a single Google Sheets tab; not great for cross-sheet or cross-app workflows.

3. Fully Automated: Let A Simular AI Agent Do It For You

Now imagine this instead: every time a CSV lands in your Drive folder or a CRM export hits your desktop, an AI agent wakes up, opens Google Sheets, cleans the data, highlights duplicates, and logs what it did.

That is where a computer-use agent like Simular Pro shines. It does not just call APIs; it literally uses your computer like a focused assistant.

A. What The Agent Actually Does

You describe the workflow once:

• Open a specific Google Sheet or create one from a new CSV.
• Apply conditional formatting rules using COUNTIF / COUNTIFS.
• Color duplicates in key columns (emails, SKUs, invoice IDs).
• Optionally, add a summary tab listing how many duplicates were found per column.

Simular then executes those steps reliably, click by click, across desktop, browser, and cloud apps.

B. Pros And Cons Of The AI-Agent Approach

Pros

• Hands-off: You define the process once and trigger it by webhook or schedule.
• Scales easily: Whether it is 1 sheet a week or 500 per day, the agent just keeps going.
• Transparent: With Simular, every action is logged and inspectable. You can see how it applied each rule.

Cons

• Requires a bit of upfront thinking to design your ideal workflow.
• Best suited once your duplicate-cleanup task is recurring, not just a one-off.

4. When To Hand This To An AI Agent

If you only audit one spreadsheet a month, manual conditional formatting is fine. But if you are:

• Importing new leads daily.
• Reconciling product catalogs across vendors.
• Cleaning client reports before sending them out.

…then you are spending strategic hours on mechanical work. That is exactly the kind of repeatable, cross-app workflow Simular was built to absorb.

Start by mastering the basic Google Sheets steps above. Once you are comfortable, capture those exact steps as instructions for an AI agent. From there, your job is to review the output, not to babysit the spreadsheet.