Repair non manifold edges does not work (saving the file)

[GiovanniSer]

I post the STL so it's possible to check
The steps to obtain the bug :
1 - I attempt to "reorient all faces coerentely"
2 - Message Filter Failure ... " Mesh has some non 2 manifold faces... "
3 - Ok I apply "Repair non manifold edges (option split vertices) .... ok, "successfully split etc etc"
4 - Now, if I attempt to "reorient all faces coerentely" ", it WORKS
5 - Ok, I save and exit
BUT
If I reload the saved file I have AGAIN the "Mesh has some not 2 manifold faces..."
DO LOOP

Internal.zip

[alemuntoni]

What format are you using to save the file? Does it happen for any format (e.g. ply)? Because STL is not a format suited for properly manage topology information, since it just saves faces, duplicating vertices.

[GiovanniSer]

I use STL, checked bin and ascii.
The ML window history says the filter worked and gives me the number of faces it corrected.
The correction was done, because after this operation, the "re-orient" no longer gives an error.
But it gives it again if I close the session and reload the file.
Thanks!

[GiovanniSer]

Instead, if I use the repair option by deleting (not split option) the bad faces, it deletes them, and saves correctly with the deletions.

[GiovanniSer]

I've done a thorough test.
I load my STL
I save it to PLY
I reload to PLY.
I correct the error in PLY
I save back to PLY and the error is gone.
I reload the PLY and I save back to STL.
I reload the STL and the same error appears again.

[alemuntoni]

The STL format is the problem. This is not a bug, is just that STL does not save meshes in indexed mode, and that is the root cause of a lot of other problems.
If you avoid the STL format (just save in PLY, OBJ, or any other format), you should not get the error again.

[GiovanniSer]

Thanks, I isolated the problem in the STL file on only 7 faces.
STL does not allow the file because two faces of seven are repeated.
Creating the PLY file, it declare wrong too, but reparing it reorders the vertices, but keeps the duplicate faces.
Basically PLY allows the overlapping of equal faces.
Unfortunately this concept is unacceptable for a correct 3D print.
I attach the two very small files, many thanks again!
compare.zip
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