Let's get the obvious out of the way. If you've paid for a tutorial, followed a tutorial offered for free, or otherwise have verbal or written permission from the artist to replicate a design, feel free! As an aside, it should be noted I'm discussing a complete design and not technique. Techniques are not protected by copyright laws. If you see a weave you like in another design, for instance, use it! But use it creatively and originally. I learned the netted bezel technique from the Eni Oken tutorial, pictured below, and used that technique with an original woven frame. Whether I used the tutorial or not to replicate the technique, it was utilized in an original design and therefore not a copy of the design. While attribution is the polite thing to do, in this instance, it's not a legal requirement.
As in my other posts on copyright, I'll reiterate that the idea you can change some variably-defined percentage of a design and still call it original is muddy waters, at best. There is no magic percentage that will protect you from copyright infringement. If you've been heavily influenced by another artist, the easiest way you can protect yourself and your work is to ask the influencing artist their opinion on the piece. Worst case scenario they say "Hey, that's a little too on the nose and reminiscent of the piece I made last month. I'd appreciate if you refrained from sharing or selling your version of my design." It happens. As artists, I understand we're often emotionally invested in the work we complete, and this response can rattle our sense of creative self, for lack of more effective phrasing. Rule of thumb: if you ask yourself "is this too close to (insert design here)", then it probably is. Ask the inspiring artist and go from there.
With that said, copying has it's place in the creative world. Artists, for thousands of years, have copied and emulated the work of masters as a study to improve their own techniques and discover their own style. I often find photographs I like online and use those as a reference to sketch. If those photographs are not open source (ie free for use), I do not share my sketches utilizing those protected images. They are just for me, to help me improve my skills, and they stay between the covers of my sketchbook. It's good to remember that not everything our fingers touch needs to go out into the world for recognition. So if you see a design on Pinterest, and you want to give it a go, I'll be the last person to say "Hey now.... whatcha think you're doing?". But I would caution, loudly, these replicas should not be shared or sold, even with attribution, without permission from the inspiring artist. If you can't determine who the inspiring artist is, don't share it. I like to err on the side of caution.
I suppose, in the end, the answer is simpler than I'd originally anticipated when I sat down to write this post.
When in doubt, ask the inspiring artist. When in doubt, ask for the opinion of an impartial third party. Can or should you copy? Yes to both. There's a place and time for it. What you do with that copy, or how you handle copyright infringement claims as a result of that copy, is another matter entirely.
In the end, folks, I encourage only one thing... stay creative and happy weaving!