Describe the bug
When you create a Step for a Partipatory_process or an Assemblie if you put an end_date prior to the start_date or start_date equal to the end_date you have the error message displayed in double.

To Reproduce
Steps to reproduce the behavior:

1. Create a Partipatory_process or an Assemblie
2. Add a step
3. set the end_date prior to the start_date or start_date equal to the end_date
4. See error

Expected behavior
The error message should be displayed just once.

Screenshots
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Stacktrace
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Extra data (please complete the following information):

• Device: Desktop
• Device OS: Windows 10
• Browser: Chrome
• Decidim Version: 0.25.2 and 0.26.2

Additional context
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