Email Duplicate Checker

Upload a blocklist CSV containing emails you have already contacted, then upload your new leads CSV. The tool loads all blocklist emails into memory, compares each lead email against the blocklist, and removes any matches. It also removes duplicate emails within the leads file itself. The result is a clean CSV ready for your next campaign.

The tool accepts CSV files only. Your blocklist file must have an Email column. Your leads file must also have an email column: the tool auto-detects common names like Email, email, EMAIL, or E-mail. If your column has a different name, select it from the dropdown after uploading.

The tool is designed to handle files with 1 million or more rows. It processes your CSV in chunks using the browser, so even very large files do not freeze or crash. Performance depends on your device: modern laptops handle 500K rows in under 30 seconds.

No. All processing runs entirely in your browser using JavaScript. Your files are read locally and never sent to any server. No email addresses, lead data, or files are stored or logged anywhere. When you close the tab, the data is gone.

The current version supports one blocklist file and one leads file per run. If you have multiple blocklists, merge them into a single CSV first, then upload. A multi-blocklist mode is planned for a future update.

All original columns from your leads file are preserved in the output. The tool only removes rows: it never drops or modifies columns. If your leads file has First Name, Last Name, Title, Email, Country, Company, Website, Industry, and Employees, those exact columns appear in the downloaded file.

After removing emails found in the blocklist, the tool also removes duplicates within the leads file itself. If the same email address appears twice in your leads list, only the first occurrence is kept. This ensures your output list is fully unique even without a blocklist.

No. Email matching is always case-insensitive. John@Company.com and john@company.com are treated as the same email. Leading and trailing spaces are also stripped before comparison, so formatting differences in exported CSVs do not cause missed duplicates.

Rows with empty or missing email values in the leads file are removed from the output. An empty email cannot be contacted and including it would waste sends or trigger errors in your cold email tool.

Yes. This tool is designed specifically for cold email workflows. Upload your master blocklist of previously contacted emails, upload your new lead batch, and download the clean unique list ready to import directly into Smartlead, Instantly, or any other cold email platform.