Hello @fyrstikken! Nice to see a post from you, this gives me the opportunity to thank you for your recent upvotes on my posts.
Also a question. What does double spend mean exactly? The person who did it bought both eth and btc for it at the same time for the same value on bittrex or something? How is it possible?
Double spend is a general term for describing an event which allows an attacker to spend the same private key multiple times, in effect creating coins from nothing. This is usually accomplished for a limited period of time, just long enough to rip off an exchange or other trader. This is most commonly accomplished on weaker networks by using a node attack in which case they create more nodes than the network currently has and for a short period of time they control the consensus of which chain is verified. This for a short period of time basically creates a secondary chain that is false, but as soon as they shut down the nodes they control, the transactions are invalidated, the original valid chain is validated again, and the "double spent" coins disappear.