I have the same issue, but with the new Panel Cloth in 5.4.
Now the first I did was exactly what was proposed. I went to blender and started “Merging by Distance”. When I used the defailt 0.0001 value, I got nothing. So I increased the threshold to a staggering 0.05 m where in fact 5000 vertices were killed. And still I get the same error in Unreal…
Something must be wrong. Either the the mesh data is incorrectly stored and gives the impression that the vertices are coincident, while in fact they are not or something else maybe some unit conversion issue.
It cannot be that vertices must not be closer than 5 cms (!) to each other for Cloth to work.