Perfect! Exactly what I wanted to achieve!
Yeah thats right. The people type in the name of their activity so this could happen. I think we will prevent this with an approval process.
Speaking of an approval process, I can filter the results easily with the "filter" statement in the pdb_list shortcode but as far as I know the pdb_signup shortcode doesen't support that anyway.
Is it possible to filter this dropdown with php code so that only approved activities/activity titles are displayed?
For that I already have a field named "approved" with the values "yes" and "no".