@Simon_Lange if the same thing happened to me, I would be just as pissed off as you so I totally understand.
For your 1st problem (forgetting one of your accounts), this is a known problem and is being worked on. A potential fix is slated for 1.2.6 to help remove corrupted accounts.
For your 2nd problem re: duplicates when re-adding your account, this is also a known problem and also being worked on and is due to come out in version 1.4 (or earlier) as file compare.
To confirm your first problem, please send us your logs.db and out.txt file to support@insynchq.com. Please include the link of this forum post.
Here’s how to find logs:
Again, I’m very sorry about your experience and we will fix the issues.