The Set error message field allows you to define the message displayed to users when they try to submit a value that has already been submitted before.

In simple terms, this message appears when Duplicate Killer detects a duplicate entry based on the unique field(s) you have configured (for example, email address, phone number, order ID, etc.). Instead of allowing the duplicate submission to be saved in the database, the plugin blocks it and shows your custom message.

Why this field matters

Clear communication improves user experience.

When a duplicate value is detected, users might otherwise be confused about why the form did not submit successfully. A well-written error message can:

• Explain what happened
• Guide the user on what to do next
• Reduce frustration
• Prevent support requests

For example, instead of a generic error, you could display:

• “This email address has already been used.”
• “A submission with this value already exists.”
• “You have already submitted this form.”

You are fully in control of the wording.

How it works in the Free version

In the Free version, this error message is global.

That means:

• The message you define here will apply to all supported forms on your website.
• Every form protected by Duplicate Killer will display the same error message when a duplicate is detected.
• You only need to configure it once.

This keeps the setup simple and fast.

How it works in the PRO version

In Duplicate Killer PRO, you get more flexibility.

In addition to the global setting, the PRO version allows you to:

• Define a custom error message per form
• Customize the message individually for different forms
• Create context-specific wording (e.g., contact form vs. registration form)

This is especially useful if:

• You manage multiple types of forms
• Each form has a different purpose
• You want tailored messaging depending on the situation

With PRO, every form can have its own dedicated error message field, giving you full control over how duplicate validation feedback is presented.

Best practice

Keep your message:

• Clear
• Short
• Action-oriented
• Easy to understand

Avoid technical language. The goal is to help users understand why the submission was blocked and what it means — not to overwhelm them with system details.

Duplicate Killer ensures clean, unique data.
This field ensures your users understand exactly what’s happening.