Learning Objective
By the end of this guide, you'll know how to configure duplicate prevention settings during lead import, understand the difference between campaign-level and workspace-level duplicate checks, and manage CRM integration scenarios where deleted leads reappear.
Why This Matters
Duplicate leads waste sending capacity, annoy prospects who receive multiple identical emails, and skew your campaign analytics. Worse, they can damage your sender reputation if prospects mark you as spam for excessive emails.
Proper duplicate management:
Protects your prospects from receiving the same campaign multiple times
Preserves your sending limits by avoiding wasted sends
Keeps analytics accurate by ensuring each lead is counted once
Prevents team overlap when multiple people manage campaigns
lemlist's duplicate prevention settings give you precise control over which leads are allowed into your campaigns.
Prerequisites
Before configuring duplicate settings:
You're importing leads via CSV or CRM integration – Duplicate settings appear during the import process
You know your import strategy – Decide whether you want strict (no duplicates anywhere) or flexible (allow duplicates across campaigns) duplicate handling
You understand your team's workflow – If multiple team members run campaigns, coordinate on duplicate policies
How Duplicate Prevention Works
During lead import, you can choose how lemlist handles duplicates:
Option 1: "Don't import leads that are already in this campaign"
Checks only the current campaign for duplicates
Allows the same lead to exist in other campaigns
Use case: You want to run multiple campaigns to the same audience with different messaging
Option 2: "Don't import leads that are already in your lemlist team (no matter the campaign)"
Checks the current campaign AND all other campaigns in your workspace
Prevents the same lead from being in any campaign at the same time
Use case: You never want to contact the same lead twice across your entire workspace
Email is the unique identifier: Duplicates are determined by email address. If two leads have the same email but different names or companies, lemlist treats them as the same lead.
Core Lesson: Step-by-Step Workflow
Phase 1: Choose Duplicate Settings During Import
Step 1: Open the campaign you want to import leads into
Go to Campaigns, then select the campaign you want to update.
Step 2: Open the Lead list and start an import
In the campaign, open the Lead list tab, then click Import new leads.
Step 3: Upload your CSV (1/4)
On Upload a CSV, upload your .csv file, then click Continue.
Step 4: Map fields and custom variables (2/4)
Review how your CSV columns are mapped to lemlist variables (and adjust if needed), then click Continue.
Step 5: (Optional) Configure AI columns use cases (3/4)
If you use AI to clean, enrich, transform, or segment imported data, select the relevant AI use cases and match any required columns from your CSV, then proceed to the next step.
Step 6: Choose duplicate prevention settings and import (4/4)
In Manage duplicates, choose one of the following:
Don't import leads that are already in this campaign (campaign-level duplicate prevention)
Don't import leads that are already in your lemlist team (no matter the campaign) (workspace-level duplicate prevention)
Then click Import to finish the import.
Phase 2: Review Import Summary
Step 7: Check the import results
After the import completes, review how many leads were:
Imported successfully
Skipped as duplicates
Failed (invalid email format, missing required fields, etc.)
Step 8: Review skipped duplicates (if needed)
If you want to confirm which leads were skipped as duplicates:
Search for a specific email in the campaign’s lead list
If the lead was skipped but you want it in this campaign, remove it from the campaign that currently contains it (if you’re using workspace-level duplicate prevention), then re-import
Understanding Campaign-Level vs. Workspace-Level Duplicate Prevention
Campaign-Level ("Don't import leads that are already in this campaign")
How it works:
lemlist checks only the current campaign for matching emails
If an email exists in the current campaign → Skip
If an email exists in a different campaign → Import anyway
Example scenario:
Campaign A: "Cold Outreach" contains [email protected]
Campaign B: "Re-engagement" imports [email protected]
With campaign-level setting: [email protected] is imported to Campaign B (different campaign, so allowed)
When to use:
You intentionally run multiple campaigns to the same leads
Example: Initial outreach campaign, then follow-up re-engagement campaign months later
Workspace-Level ("Don't import leads that are already in your lemlist team (no matter the campaign)")
How it works:
lemlist checks the current campaign AND all other campaigns in your workspace
If an email exists anywhere → Skip
Example scenario:
Campaign A: "Cold Outreach" contains [email protected]
Campaign B: "Re-engagement" tries to import [email protected]
With workspace-level setting: [email protected] is skipped (already in Campaign A)
When to use:
You want to ensure no lead receives multiple campaigns at once
Multiple team members manage campaigns and you need coordination
You want strict duplicate prevention across your entire workspace
Practical Application / Real-Life Example
Sales Team Prevents Cross-Campaign Duplicates
A B2B sales team has 5 reps, each running their own campaigns targeting similar audiences (SaaS CMOs). Without duplicate prevention, the same CMO could receive emails from 3–4 different reps.
Their solution:
Enable "Don't import leads that are already in your lemlist team (no matter the campaign)" for all imports.
Workflow:
Rep 1 imports 500 leads to "Q4 Outreach Campaign"
Rep 2 imports 600 leads to "Product Launch Campaign"
lemlist checks: 150 leads from Rep 2's import already exist in Rep 1's campaign
Result: 150 duplicates skipped, only 450 new leads imported
Outcome:
No CMO receives emails from multiple reps
Clean, coordinated outreach across the team
Sender reputation protected
Key takeaway: Workspace-level duplicate prevention is essential for teams to avoid internal competition and prospect confusion.
FAQs on Lead Import and Duplicates
Why do deleted leads keep reappearing in campaigns?
Root cause: CRM integration refreshes and re-imports leads from the source list.
How it works:
You delete a lead from your lemlist campaign
Your CRM integration runs its periodic refresh (e.g., daily sync)
The lead still exists in the CRM's source list (e.g., a filtered view or segment)
lemlist re-imports the lead because it's still in the CRM source
Fix:
Remove the lead from the CRM's source list (the filter, segment, or view that feeds lemlist)
Or exclude the lead in your CRM using tags or status (e.g., mark as "Do Not Contact")
Or adjust the CRM integration settings to exclude certain statuses or tags
Best practice: If you delete a lead from lemlist, also update their status in your CRM to prevent re-import.
How do I ensure new leads are added to my campaign automatically?
For CRM integrations:
New leads added to your CRM's filtered list or segment will sync automatically during periodic refreshes
Check your integration settings to confirm sync frequency (daily, hourly, etc.)
Verify the CRM filter or segment is correctly configured to include new leads
For manual imports:
New leads won't automatically appear, you need to manually import updated CSV files
Consider using Zapier or CRM integration for automatic syncing
Troubleshooting
Issue: All my leads are being skipped as duplicates
Root cause: All leads already exist in campaigns, or duplicate setting is too strict
Fix:
Check which campaigns contain those leads (search by email in other campaigns)
If the leads are in old/completed campaigns, remove them from those campaigns first
Or use campaign-level duplicate prevention instead of workspace-level if you intentionally want duplicates across campaigns
Issue: Duplicate leads are being imported despite settings
Root cause: Emails have slight variations (extra spaces, different capitalization) that lemlist treats as different
Fix:
Clean your CSV data before importing:
Remove extra spaces
Use consistent capitalization
Use a data cleaning tool or Excel formulas to standardize emails before import
Issue: I want to move a lead from Campaign A to Campaign B
Root cause: Workspace-level duplicate prevention blocks the lead from being in both campaigns
Fix:
Remove the lead from Campaign A (or wait for them to complete that campaign)
Import the lead to Campaign B
Workspace-level duplicate prevention will no longer block them
Issue: CRM integration keeps re-importing leads I deleted
Root cause: Deleted leads still exist in the CRM source list
Fix:
Update the lead's status in your CRM (e.g., mark as "Unsubscribed" or "Do Not Contact")
Adjust your CRM filter or segment to exclude leads with that status
lemlist will skip them during the next sync
Optimization Tips
Use workspace-level prevention by default: Unless you have a specific reason to allow duplicates across campaigns, use "Don't import leads that are already in your lemlist team (no matter the campaign)". This is the safest setting for teams.
Clean data before importing: Standardize email formats and remove extra spaces. This prevents false positives where lemlist treats slightly different emails as unique.
Coordinate with your team: If multiple people import leads, agree on a duplicate prevention policy and document it. Everyone should use the same setting to maintain consistency.
Review skipped leads: After imports, investigate which campaigns duplicates are in and whether they should be removed from old campaigns.
Tag leads by source: When importing, tag leads with their source (e.g., "Q4 LinkedIn Scrape"). This helps you identify where duplicates came from.
Archive completed campaigns: Once a campaign is done, archive it. This keeps active campaigns clean and reduces the chance of duplicate conflicts with old campaigns.
Use CRM integration wisely: Carefully configure your source filters to exclude unsubscribed, bounced, or "do not contact" leads. This prevents unwanted leads from re-importing.
Export before bulk operations: Before making bulk changes (deleting old campaigns, removing duplicates), export your lead lists as backups.
Set up sync schedules: If using CRM integration, align sync frequency with your campaign pace. Daily syncs work for high-volume campaigns; weekly syncs suffice for slower campaigns.