I could see people were copying but it was hard to prove it 100% but this was a good way to do that!

Can I suggest you add a second rule to the giveaway? Add something like "as well as commenting on your guess also write one sentence about...." Just pick something different each time. Then people have to actually read the post to see what they have to write about. For example "write one sentence about your favourite rock song", "your favourite card in Rising Star and why", etc.