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# How can I prevent duplicate submissions in my system by checking against existing values in a database or data structure?

## Prevent Duplicate or Invalid Values with Custom Validation

Want to ensure users don’t submit a value that already exists in your form’s database? Follow these steps to set up custom validation:

> ⚠️ **Important Limitation:** Currently, the "Unique Submission" validation rule only supports checking **one field at a time**. If you combine multiple fields using "AND" logic, the system will check each field individually across all submissions, rather than checking if the specific combination exists in a single record.

### ✅ Step-by-Step Guide

1. **Log in** to your account.  
2. Go to the **Forms** section.  
3. Select the form you want to edit.  
4. Find the field you want to validate and click the **⚙️ settings icon**.  
5. On the right panel, open the **Custom Validation** section.  
6. Select the input field under "Operator," then click **Add condition**.  
7. Choose either:  
* **"Exists in submissions"** – to check if the value already exists (e.g., to block duplicates), or  
* **"Does not exist in submissions"** – to allow only new/unique values.  
8. Add a custom **error message** that will display if the condition is not met.  

![Custom Validation Example](https://storage.crisp.chat/users/helpdesk/website/-/5/d/9/1/5d91dadd0b88a000/image_w4dack.png)

9. Click **Publish** to save your changes.

### Example

Use this to prevent duplicate email addresses, usernames, or IDs from being submitted more than once.

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### 📝 Supported Fields for Custom Validation

You can apply custom validation to the following field types:

* **Title**  
* **Email**  
* **URL**  
* **Phone number**  
* **Rich Text**  
* **Number**  
* **Select**  
* **Multi Select**  
* **Status**  
* **Date**  
* **User**  
* **Relation**  

> ⚠️ Note: Some complex fields (like **Relation** or **User**) may require additional setup or reference to linked databases.