In UI design, where is the line between copying someone and getting inspiration?

I could see this question in two aspects: the designer’s POV and the audience’s POV

• The designer’s personal POV about replicating and modifying depends on her philosophy.
• Is it OK to copy just to get it done? Is it OK to spend time to reinvent the wheel? What will the designer learn from it?
• What will the designer’s audience think about the practice of copying visual designs? What will the audience feel about a familiar experience in the design?

There are many different considerations when someone seems to have a similar UI design. I could see the thin transparent line all over the spectrum.

Originally published in Quora