As you can see, there are 4 vertices here, 4 faces, and 6 edges.
I’v tried “remove doubles” and all the clean mesh menu options.
Is there any way to remove them except one by one?
Note the face indicators (large dots) on the edges, bad. Remove them manually and make sure you don’t create them in the future by actually taking some care when modelling.
You can have overlapping faces (avoid) but when merging vertices can still have more then one face.
This is not my model, This is a model I got from solid work and I try to use it in blender. So I can’t change anything about it’s creation.
If the mesh you got from Solidworks was that plane split into four parts, I think you might’ve spent a bit of unnecessary effort as you could’ve just done an inset operation followed by scaling the middle face down to 0 and then removing the doubles.
If it’s not, can we see what the whole object looks like?