Update of certification leads to an add while editing the activation Date

Step to reproduce :

1. Having an already targeted certification (Google PDE for my concern)
2. Edit it and say "Got it !"
3. Save and see: so far, so good
4. Edit the end validation date and save. Always good
5. See that the starting date is always the targeted one, so edit the start validation date
6. After editing and save, the certification appears twice, one with the old targeted date as a starting point of validation, the second with the new edited starting date.

Start by reproducing the certification edit flow described in the issue, focusing on how targeted certifications and validation dates are loaded and saved. Trace the handling of the starting date after the end date is edited; done means changing that date updates the existing certification without creating a duplicate.