Say goodbye to that money. Gone. Sorry about the bad luck.
This is how double spend works. They set up a transaction (first spend), then when it falls as unsuccessful, they spend the same money on a different address/transaction (second spend).
The first transaction 'didn't happen' like the guy said above. Therefore, you are out of your money and fucked.
Bitcoin is very complex. Most people just get robbed and then quit. Please be careful. Anyone can just log in to your computer and get all your savings in an instant.
True, it's quite complicated, up until now the only thing I know is how to send and receive money on my online wallet. Still confused with the technical stuff. Does this mean he wouldn't get his money back? That would suck
Do you think this is possible to avoid? I'm currently learning stuff like Photoshop and later I'll take some CAD seminar. Planning to freelance IRL and probably also advertise my service OL in this forum. This incident made me uneasy. What if I already sent them the finished project and then the payment I received ended up like OP's transaction? If the client is like half-way accross the globe, obviously I wouldn't be able to sue to them (or at least show up outside their office to pester them to give me my payment) 
You must be very smart and very fast to get your bitcoin into your second wallet before the original sender double spends your money. If you don't understand where your keys to your second wallet (protected) are, you won't do this right and you'll lose all your money. You can't just leave your bitcoin floating out there where the other guy can spend it again.
Please be careful. You can lose everything in an instant. One mistake, and your money is gone forever.