Legal Question - Duplicating a Project (sort-of)

I'm the owner and sole employee of a site development company. I recently built a fantastically complicated system for a local company, and soon after I started building it I realized that it would be an incredible thing to sell to other businesses in the same industry. The product wouldn't be a carbon copy; it would address the same needs in a similar way, but it would be a more universally accessible tool. New companies that wanted it would all log in to one central system for a subscription fee.

I mentally realize that the two products would have vastly different code, but I can't help the nagging feeling that this business owner would likely try to sue a chunk out of me for making the new version. He would almost have a leg to stand on; it was his input that determined what the features needed to be. But then, that knowledge isn't unique. It's just that he was the only guy in the industry that I was talking to.

We have no exclusivity contracts between us. He owns the original code that I wrote, but the new stuff would be entirely different. It would merely meet the same needs.

Has anyone dealt with this sort of issue before? I'm excited about the prospect of building this thing, but I'm leery of getting a beat-down, too.

Really you probably want to talk to a real attorney and have available all documents from your contract to any correspondence. I would say he probably would have a case as you developed a system for him which was essentially his idea and now you're duplicating it. He might even well have filed for a patent, which you wouldn't need to know about as a work-for-hire developer.

Thanks for the heads-up. I'll run this past a lawyer and see what's what.

Thanks, medialint!