With Blender 2.8 a number of editing functions previously available in the Tool Shelf have been reorganised or relocated so they make more sense contextually whilst working, one such tool being Edit Mode’s Remove Doubles (RemDub) that collapses together/merges coincidental vertices, those within an adjustable distance or degree of tolerance.
Design note: in moving Remove Doubles to a context sensitive main menu, Mesh, the Toolbar space otherwise occupied is freed for more immediately useful tools and options whilst working in Edit Mode.
To access Remove Doubles in Edit Mode, click the Mesh menu upper-right of the 3D View. Towards the bottom of the list click Clean Up then Remove Doubles from the additional options shown – Mesh » Clean Up » Remove Doubles.
Important: depending on the version of Blender 2.8 used, Remove Doubles is renamed Merge by Distance. The feature is still accessible from Mesh » Clean Up » … and still functions the same way.
Design note: to adjust the tolerance during the action Merge Distance can be increased or decreased in the Remove Doubles overlay that appears bottom-right – adjusting the value keeps the action live until another element is clicked or Edit Mode is exited.
To access Remove Doubles in Blender 2.8 click Mesh » Clean Up » Remove Doubles.