This was quite common back then. I remember fifty five years ago a news report of Egyptologists taking apart a papyrus breastplate from a mummy and finding an unknown Greek play written on the pieces. It was the story of a boy and girl separated when young, then as adults coming together, falling in love and soon to get married. The Egyptologists were in suspense trying to find how the play ended. We never found out.
Old Story - Boy meet Girl, Boy loses Girl, Boy and Girl
meet again ……..
I forgot to mention they were brother and sister.
So Shakespeare was a plagiarist?