Outlook's Clean Up vs removing duplicates

Duplicate emails usually come from importing the same PST twice, merging overlapping backups, or a sync that went wrong. Outlook’s Clean Up command sounds like the fix, but it is built for something else.

What Clean Up does

Clean Up (on the Home tab: Clean Up Conversation, Clean Up Folder or Clean Up Folder & Subfolders) removes redundant messages in a conversation: earlier messages whose full text is already quoted in a later reply. If Anna’s question is quoted in full in Ben’s answer, Clean Up can remove Anna’s message and keep Ben’s.

By default the removed messages go to Deleted Items. You can change that, and which messages Clean Up keeps, under File › Options › Mail › Conversation Clean Up.

Why it does not fix duplicates

• It works by conversation. It compares messages within a thread and looks at whether one is quoted in another. Copies left by importing the same file twice are not “replies quoting each other”, so they are not what it is designed to find.
• It can remove messages that are not duplicates. An earlier message whose text appears in a reply is removed even though it is a different message, so its own attachments or recipients may matter to you.
• It changes your mailbox in place. There is no report of exactly what was moved, beyond what you find in Deleted Items.

Clean Up is a reasonable way to shrink long conversations. It is not a duplicate remover.

Remove true duplicates

Remove duplicates looks for copies of the same message:

• Exact copies: every detail identical, the safest choice.
• Same message: same sender, recipients, time, text and attachments, even if two programs stored it slightly differently.
• Inside each folder, so a message you filed in two folders keeps both, or anywhere in the file, for merged backups.

It counts the duplicates for free before anything is written, keeps the first copy of each, writes a new PST without the rest, and lists every removed message in the report. Your original file is never changed. It runs in your browser: nothing is uploaded.

Avoid duplicates next time

• When importing a PST in Outlook, choose Do not import duplicates (import a PST).
• When combining backups, merge them first and remove duplicates afterwards: see merge PST files without duplicates.

Frequently asked

Will Remove duplicates touch my mailbox? No. It works on a PST file. To clean a mailbox, export it to a PST, remove the duplicates, and open or import the clean copy.

Can I see the duplicates before removing them? Yes. The free check shows how many there are and the largest groups before you decide.