If you get someone to make some software for you,can the programmer copy your software? Because the programmer will know your idea now.
In the USA you have the DMCA which is a fairly tough law that protects your copyrighted works. Elsewhere you need to study each case and write an intelligent contract that includes a non disclosure agreement so they can't post your stuff on forums and other clauses to disallow them to employ the code on other systems. Of course there's nothing you can do about them secretly selling the idea online, but that's just the nature of software since 1950....
Go with some one reputable. But seems like you have already shared your idea and its worrying you. right?
I have seen people stealing the idea and launching their own products. I think this is the same case with copycats. The developer can rewrite the entire code and I guess nobody can have a copyright/patent for an idea.
It all comes down to your contract and/ or the country who's law governs the interaction. All our contracts clearly spell out who owns the rights to what and specify that the contract is governed by UK law. If you require specific clauses in the contract such as non-competition then these should be discussed prior to signing on the line.