Well, basically I wanted to move from bitcoin-qt client to multibit-client. So I exported the privkeys from bitcoin-qt (B+C). Multibit came with it's own start-address (A). Upon importing B, I had W_1 (A+B) but it displayed transactions and the end-sum wrong. I assumed, because C was still missing, so I created W_2, this time importing B+C, so I got A+B+C. Now it was even more screwed up and displayed double my correct balance. W_3 and W_4 were just created to test further and try to find the error.

I can reset the blockchain and run a replay once from the beginning to see, if it clears things up. Or just delete W_1 and W_2 and go with W_3 and W_4 now.