Doesn’t Mailchimp remove duplicate addresses itself?

An address repeated exactly, yes — Mailchimp imports it once. Two spellings of one address, differing in capitals or a space, are two addresses to it, which is what the duplicate remover here finds.

Why were some addresses rejected by the import?

Usually a space before or after the address, or an accented letter before the @, both of which Mailchimp’s help lists as problems. Find and replace with a pattern clears the spaces in one pass.

Can I import an Excel file into Mailchimp?

Mailchimp takes CSV or tab-delimited text, not a workbook. The Excel converter here turns an .xlsx sheet into a CSV with every value kept as text.

Which copy of a duplicate is kept?

The one with the fewest blank fields, in the settings the button opens — usually the row with the name and the signup date filled in. First and last are there too if you prefer.

Are omar+offers@example.com and omar@example.com the same subscriber?

They reach the same inbox, but they are the addresses the person gave, so they stay separate unless you turn on the option that ignores the part after a plus.

Is my subscriber list sent anywhere while I clean it?

No. Duplicates are found and spaces cleared inside this page, on your own device; the list reaches Mailchimp only when you upload the cleaned file yourself.

All guides

Prepare a CSV for Mailchimp: one address per contact, and no stray spaces

Mailchimp needs one column of email addresses, one address per contact, in a CSV or tab-delimited text file; by Mailchimp’s own help, a space before or after an address is enough to make it fail. It merges an address repeated exactly, but GRACE@example.com with a trailing space and grace@example.com are two different strings to anything that compares text. Finding those, and clearing the spaces, is the preparation that makes the import clean.

What Mailchimp checks

Mailchimp’s help asks for a file with a column of email addresses and one address for each contact, saved as CSV or tab-delimited TXT — not an Excel workbook, not a vCard. It flags addresses with spaces before or after them, and it can only take accented letters like é after the @, not before it.

An address that appears twice exactly is imported once; Mailchimp does that itself. What it does not do is decide that two differently written addresses are the same person, and that is where a list assembled from two sources goes wrong.

The duplicates Mailchimp cannot see

The button above opens the duplicate remover already set up for this: comparing on the email column, ignoring capital letters and stray spaces, and keeping the most complete copy of each person. On the sample list, the two Ada rows become one and the two Grace rows — one in capitals with a trailing space — become one. Your removed rows are listed separately, so you can see exactly what went.

The two Omar addresses stay two, deliberately. omar+offers@example.com may be exactly how that person chose to sign up, and treating the part after a plus as disposable is an option you switch on when you mean it, not a default.

If your own list names its address column something else — Mailchimp’s own exports call it “Email Address” — choose that column in the settings instead.

Then the spaces, and the rest of the file

The copy that was kept can still carry the stray space. Find and replace on the email column, with pattern matching on, turns ^\s+|\s+$ into nothing: it takes the spaces off both ends of every address, shows how many cells change, and touches no other column.

If the list is an .xlsx workbook, the Excel converter here writes the sheet as a CSV with every value as text; an old .xls has to be saved as .xlsx or CSV from Excel first. Columns Mailchimp will not use — internal ids, notes — can go with the column remover, so the import screen has only the fields you mean to map.

The sample file

email,name,signed_up
ada@example.com,Ada Fictional,2025-01-04
ada@example.com,Ada Fictional,2025-01-04
GRACE@example.com ,Grace Example,2025-02-11
grace@example.com,Grace Example,2025-02-11
omar+offers@example.com,Omar Pretend,2025-03-02
omar@example.com,Omar Pretend,2025-03-02
priya@example.com,Priya Sample,2025-03-19

Obviously made-up data, small enough to read. Download it and drop it on the tool to see the fix before trying your own file.

Questions

Doesn’t Mailchimp remove duplicate addresses itself?: An address repeated exactly, yes — Mailchimp imports it once. Two spellings of one address, differing in capitals or a space, are two addresses to it, which is what the duplicate remover here finds.
Why were some addresses rejected by the import?: Usually a space before or after the address, or an accented letter before the @, both of which Mailchimp’s help lists as problems. Find and replace with a pattern clears the spaces in one pass.
Can I import an Excel file into Mailchimp?: Mailchimp takes CSV or tab-delimited text, not a workbook. The Excel converter here turns an .xlsx sheet into a CSV with every value kept as text.
Which copy of a duplicate is kept?: The one with the fewest blank fields, in the settings the button opens — usually the row with the name and the signup date filled in. First and last are there too if you prefer.
Are omar+offers@example.com and omar@example.com the same subscriber?: They reach the same inbox, but they are the addresses the person gave, so they stay separate unless you turn on the option that ignores the part after a plus.
Is my subscriber list sent anywhere while I clean it?: No. Duplicates are found and spaces cleared inside this page, on your own device; the list reaches Mailchimp only when you upload the cleaned file yourself.