Why does it keep installing duplicate afcads for FSDT airports I already have installed
It doesn't, check better your folder, there are no duplicates.
If you modified the AFCAD, it will download the most current version of the AFCAD, but it will also make a BACKUP of your old file, with date/time of the installation, with a .BAK extension, so it's not a duplicate because is not seen by FSX.
Are the duplicates the latest parking and runway assignments
They are the most current version as made by us, which usually fix small problems reported by users.
if not I want my custom afcads back.
The whole point of making a backup, is that you can restore your customization by simply copying it over the standard file. Of course, it's best if you base your modifications with a recent version of our AFCAD, otherwise you'll get your modifications back, but you'll lose our recent fixes.