Hey Sjaak,
This issue has been resolved and is included in the IBM Connections 3.0.1 FixPack (3.0.1.1):
LO66226-Duplicate header error when downloading a file using Chrome.
Fix list can be found here:
https://www-304.ibm.com/support/docview.wss?uid=swg21587364
ReadMe:
https://www-304.ibm.com/support/docview.wss?uid=swg21584430
Brian