The convenient Blender 2.7 feature to remove duplicated vertices “Remove Doubles” has disappeared from the Blender 2.80 menu. In 2.8 it was renamed to “By Distance” and moved to the Vertex – Merge Vertices menu.
In Blender 2.83 this menu was moved to the context menu, activated by the right mouse click. And quick call by the “m” hot-key.