Wow Thanks for your fast answer mazi.
1. Is is browser related?
Ist there a way to check the browser version at the time a survey is taken?
2. Does the user maybe press the back button or something?
Maybe, would this be a problem?
3. Do you use tokens?
Yes we use unique tokens for every paticipant. Token is identical in the duplicate entries.
4. Are there any differences between the two data sets?
nope... completely identical
---EDIT---
Except the timestamp for "invitation sent" is different. This is interesting...
---/EDIT---
--- EDIT2---
The timestamp for completing the survey is the same...
If somehow 2 Identical Users are created, with the same token. Would this trigger a doupilcate survey result, as the user fills out the survey?
----/EDIT2----
5. Is there anything suspicions at the server's error log?
Everything seems normal
6. Are any other warnings shown?
no
7. Did user use the "save" feature to return to the survey later?
May, be... is there a way to check this?
8. Does the problem only show up at this one survey?
Problem only shows up in this survey. But not right from the start. The first 900 were ok
9. Did you try to build the survey from scratch (or parts of it)? If so, could you reproduce the problem?
No i didn't but we ned the database to span at least one year.