Merel,
You can do the following in Outlook:
- create a new folder with Contacts type in Outlook
- go to File -> Open -> Open Outlook data file -> select an old PST file on the disk (this will open it in Outlook)
- find there Contacts folder and copy contacts to created folder. You can do it via copy-paste
- press right mouse key on the old PST file and select close to detach it from the Outlook
- repeat steps for each old PST
- copy contacts from current PST
- remove duplicates in the folder using
Remove Duplicate Contacts utility
- you can also try to remove unused contact using
Remove Unused Outlook Contacts utility
- delete some contacts manually if needed
- If you are satisfied with the result, delete contacts from your current contact folder and copy from this folder to it.