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Find Duplicate Outlook Emails If They Were Not Detected As Duplicates

Sometimes in the process of finding and removing duplicate email messages and other Outlook items, a situation arises where seemingly identical messages are not detected as duplicates. It can be difficult to see the differences unless you use special tools.

Duplicate emails are a common disturbance in Microsoft Outlook. It is easily fixed if you eliminate the reason for the email duplication in time and get rid of duplicate messages that have appeared in your mailbox. However, sometimes, users complain that they can't delete duplicate emails in Outlook with built-in tools or even with automated professional software. They see the messages that look exactly the same, but the program cannot find any duplicates.

Let's see why this happens and how to solve the issue.

What are duplicate emails in Outlook

First, one must distinguish between real duplicate Outlook messages and those that only seem duplicates. Duplicates can be identified in different ways, for example, by comparing Message-ID or hash values of other message fields. We must remember that emails with the same Message-ID are duplicates, but they may not be exact copies. And if you compare only one or two values of message fields, you can delete messages which do not look alike even visually.

Therefore, the more fields you compare, the more accurate the comparison, but also the higher the probability that one of them may be slightly different. And the automatic duplicate email finder may not consider the emails duplicates.

What is Outlook Message-ID

Message-ID or Internet message ID is the unique identifier for an email. When you send the email, it is automatically set by the mail client or server in the email message header. Since Message-ID is unique, when it matches several messages, they are recognized as full duplicates, even if the messages have been changed.

How to find Message-ID in Outlook

The value of Message-ID can be found in the Internet headers of Outlook messages. To show Message-ID in Outlook:

  1. Double-click on the desired message to open it in a separate window;
  2. Go to File > Info > Properties;
  3. The Message-ID is shown in the opened window's Internet headers field. Sometimes, this box might be empty. Then such a message will not be recognized as a duplicated email, even if it is repeated in the same folder.
Message-ID header in Outlook

What is the hash number of the email message

Hash is a special number unique to a certain value. It is calculated in a particular way for a specific data set. These can be texts, images, video files, or other data. Here, it is the identifier of some part of a message, such as an email address or message text.

If the hashes are different, the message fields are also different from the program's viewpoint. But a text change may occur due to an antivirus stamp, mail system malfunction, etc. Meanwhile, the messages may be identical in everything else.

How to compare Outlook emails to find and remove all similar messages

The advanced versions of utilities for checking, tagging, and removing duplicate messages in Outlook provide many features to compare emails by the defined fields. If you already know what fields are different, you can configure the utility to ignore them and consider the messages as duplicates of other identical fields. What if all the message fields you see seem identical, but messages are still not auto-detected as duplicates? Here, you need to determine which fields to ignore.

The diagnostic mode of the Duplicate Outlook Items Report will help you to find the subtle differences between similar emails and correctly set up the Outlook deduplication. To discover why duplicate emails in Outlook are not identified as duplicates, run the "Outlook Duplicate Items Report" highlighting these emails.

Running the report on Outlook duplicates

As well as our duplicate removers and Outlook duplicate finders, the report on Outlook duplicates provides two main modes for comparing duplicate emails.

The default comparison method

By default, the program compares Message-ID Internet message headers, if any. If this field is missing from two duplicate messages, then the program will compare the hash values of the important fields. But if one message has a Message-ID and the other does not, the program will not detect them as duplicates.

To see the differences between emails, enable the Show diagnostics information setting on the General tab. You can immediately see that the selected messages have different fields.

Diagnosing Outlook duplicate differences

The method of comparing selected fields

If you enable the Use custom fields to compare setting on the Messages tab, the program will compare the values of the selected fields. In the final report, we will see hash values that clearly show the difference in the field values.

Even in this case the messages may be not recognized as duplicates. With the diagnostics mode, you can see the details. They are almost the same but of different text bodies. If you consider that messages are identical, disable the comparison of inappropriate fields.

Differences in similar Outlook emails

Now, you can build the report on a folder or run the utilities for finding or removing duplicate emails using the same configuration to find the duplicates.

Warning! When permanently deleting duplicates, disabling comparison fields can be dangerous! When removing duplicates among many messages, there is a risk of mistakenly eliminating emails that are different.


If you find multiple copies of the same emails in your Outlook inbox or other Outlook folders that are too many to clean up manually, the surest solution is to use an automated remover tool to delete duplicates.

Because the automatic instruments use unique message identifiers not always visible to email users, you may sometimes think that they don't work properly.

These programs have flexible, advanced settings that allow them to be adjusted for your situation. And a special tool for identifying similar messages will detect unseen differences and tell you how to configure the utilities to find and delete duplicate emails. This way, you can remove all redundant messages in Outlook bothering you.


How to identify duplicate emails in Outlook?

The email is a real duplicate if all its properties and fields are identical.

Why are duplicate emails in Outlook not found?

If an application you use detected no duplicates, but you still see them, it doesn't necessarily mean an error. You may not be aware that the same messages have different fields. It is easy to check with the Duplicate Outlook Items Report utility as described in the article.

Can I decide myself what emails are duplicates?

Yes, you can set up the advanced versions of our tools for duplicate detection to retrieve all the emails with the same required fields.

Does the program search Outlook duplicate emails in different folders?

Yes, you can choose as many folders as you want to search within. By default, the program compares messages in each folder separately. Enable the Search duplicates across folders option to look for duplicates in all folders at once.

How to fix duplicate emails in MS Outlook?

Read the article about duplicate troubleshooting to remove the cause and how to deduplicate Outlook mailbox to fix the message duplicity in Outlook.

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