Doubling of attributes after a failed saving attempt.

**Environment**
1. Version 2025.10.3
2. Integration TYPO3, 13.4, PHP 8.2
3. System Linux

**Describe the bug**
When you edit a product, and saving of the product fails, some of the attributes get doubled.

**To Reproduce**
Steps to reproduce the behaviour:
1. Install a new vanilla Aimeos and install the demo data
2. Create a new Product.
3. Go to the characteristics panel
4. Add the `interval` attribute to 'hidden', with the value `Demo: One month`
5. Add the `interval` attribute to 'hidden', with the value `Demo: One month`, again. This will make the saving of the product fail.
6. Save the product

The saving attempt fails, as it should.
Now you have the `interval` four times, not just two times.
The amount of attributes is doubled every time you try to save it.

**Expected behavior**
The saving attempt fails.
The `interval` attribute is displayed two times, not four times.

**Screenshots**

**Additional context**
We are doing a sanity check when we save a product, and display an error message when the editor does not provide the needed data. And every time the check fails, all attributed are doubled.

This is not about having the same attribute with the same value twice. This is about the product form displaying the same attribute multiple times after a failed save attempt.

Tobi