When you're dealing with Open Hardware, that question has a clear and specific answer: you're free to use the design however you want, which includes making and selling flat-out copies of the original. It's a point of courtesy in the community to give credit where it's due, and good citizens of the OH community have a tendency to hunt down the people whose designs they want to use and ask for a blessing before releasing their own version of the work.. pretty much what you've done with your friend's design.
Bottom line though, 'Open' means 'Open' and anyone who releases a design under those terms had darn well better know and accept the consequences.
When it comes to parallel development of non-Open hardware, well, that happens. If you didn't buy someone else's product, reverse engineer it, then hork out a clone simply to make a fast buck off someone else's R&D, your conscience should be clear.
. . . if at the time of conception or design you don't know about previous peoples work, would you worry about being called out to have plagiarized their work/idea?
No. By definition, it's impossible to have copied something you didn't know about at the time.
If someone wants to accuse you of ripping off their work, they're making a claim. That means they have an obligation to support that claim, and the first thing they have to do is prove that you actually did see and copy the work in question. Just saying, "yours is like mine so you must have copied me" isn't good enough.
In this case, the term 'e-pet' makes me think of the Tamogotchi, which Bandai released in 1996. It's about sixteen years too late to worry about who came up with that idea first, and "both works in question derive from a common body of work that the plaintiff does not own" is a standard defense against copyright/patent claims.
Instead of focusing on the similarities between your kit and anything else in the same space, focus on the things that make your idea unique. That's your 'market discriminant', and is what makes your kit worth buying instead of another. Not having a clear market discriminant would stop me from releasing a product faster than "people might say I'm copying" would.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.