Describe the bug

When I receive an invitation as an already registered participant I have the flow of "Finish create your account", although I've already created and had a nickname/password/etc.

To Reproduce
Steps to reproduce the behavior:

1. Register and confirm a participant account
2. Log in as administrator
3. Go to a Admin dashboard -> Participatory Process -> Process admins
4. Click the button "New process admin"
5. Fill the email and username of the participant account of Step 1
6. Click in the "Create" button
7. Log out
8. Go to the participant account email. Search mail with subject "You've been invited to manage Decidim"
9. Click in "Accept invitation"
10. View error with "Finish create your account"

Expected behavior

I expected to go directly to the admin panel to this process (and see the admin ToS for example)

Screenshots

Extra data (please complete the following information):

• Device: Any
• Device OS: N/A
• Browser: N/A
• Decidim Version: 0.24.dev
• Decidim installation: Local