Select all the vertices in edit mode and then
v2.79 and below
click spacebar in viewport and type in "Remove Doubles" in the search.
v2.8+
click F3 in viewport and type in "Merge by distance" in the search.
(or)
You can use this software called "Instant Mesh" by wjakob by loading the exported mesh file into this software and get a clean mesh.
link https://github.com/wjakob/instant-meshes
Hope this helps you!