Tracking whether a respondent has previously participated, and if so, do some branching.

Hi.
Step 1
I want to create a base survey (with an appended Unique ID - UID) such that when a participant opens it in a web browser, it checks if the UID in the URL already exists.
Step 2
If the UID already exists it then it randomly picks one of three possible subsequent surveys to run (e.g. A, B or C) that have not yet been completed.
In order for this to work I need to have a way to store the UID and whether survey A, B or C has been completed.
so my logic is this:
When the base survey is first run, I check if data for that UID exists. If it does then all subsequent answers update that file.
If it does exist, randomly select survey A, B or C. For each survey, there will be a corresponding var indicating whether that survey has been done (e.g. varA, varB or varC). Loop through each one until a non-complete survey has been found and then quit when that survey is done (marking the corresponding var as complete).
If it does not exist, then simply randomly pick on the 3 surveys to run.
I know how to do the randomisation bit etc. What I need to know is, is it possible to check if a UID has already got data, and if so can it be updated in this way?
NB. I want to use the same link for each participant (differentiated only by UID) rather than have three separate links. The rationale for this is that I am setting up a multipart study in EMS and there's no way to randomise the multipart order that I know of.