There is a rule of thumb in the knife and tool world that when you are about to make something that might be close to someone else's design that you contact the other maker out of courtesy and ask if the design or even elements of the design are too close to theirs. I have done this with other makers and people have come to me and shown the same courtesy. It is the polite and respectful way to go about it.
Usually the concern is nothing to get excited about and the prospective maker was being a little too sensitive and overreacting to a specific element that they thought was similar to the original. Often there is no cause for concern and the original maker gives their blessing and seal of approval to carry on. But if something does prove to be a little too close then the parties have a chance to work out the differences and avoid public embarassment and confrontations.
Recently I came upon an instance where there was a disagreement but we have resolved our differences to the satisfaction of both parties. Lessons learned on both sides and we're both the better for it. :)