I am not so sure, if I understood the setting correctly. Here's another reading :)
If oyu have quite separate studies and want to ask for the personal data only once (or once a year...), then you need some mechanism to recognize a respondent. If it's already on the list, you can (a) insert the data from the database for contents and using preset(), or (b) you can just store a reference in the data set to match the actual data later.