New, duplicate question is:
I chose a different duplicate to close. When I tried adding the second duplicate to the question, the SE software blocked my edit, stating that the title (which the OP typed in) already exists.
Note that the "Duplicate Title" error appears, even though the new user typed a period at the end of the title. Such periods are supposed to be removed automatically by the system. It is probably the removal of the period from my edit that caused the duplication, so the real bug is this didn't happen for the OP.