Inspiration : the process of being mentally stimulated to do or feel something, especially to do something creative
Plagiarism : the "wrongful appropriation" and "stealing and publication" of another author's "language, thoughts, ideas, or expressions" and the representation of them as one's own original work
One can be inspired to write similar concept which matches someone else's concept too. Example : I wrote a story about two doctors who fall in love while working together. If someone else's writes a story along the similar concept then that's being inspired. One does not have sole ownership when it comes to concept.
However, if somebody borrows my plot without my permission and posts it as theirs then that's counted as plagiarism. Even if you change the name, or certain words or sentences it's still plagiarism which is wrong.
Keep in mind IF does not tolerate plagiarism is any form and strict actions are taken if someone is found guilty.