isoSurfaceTopo not passing through ACMI

Run oscillatingInletACMI2D tutorial with

isoSurfaceTopo
{
    type surfaces;
    libs ("libsampling.so");

    enabled         true;
    writeControl    timeStep;
    writeInterval 1;
    interpolationScheme cellPoint;
    surfaceFormat   vtk;

    // Fields to be sampled
    fields
    (
        p
        U
    );

    surfaces
    (
        isoSurface1
        {
            type            isoSurfaceTopo;
            patches         ( ".*");
            isoField        p;
            isoValue        0.1;
            interpolate     true;
            triangulate     false;
            regularise      cell;   //diagcell;
        }
    );
}

and the isosurface does not pass through the acmi. This is due to the duplicate faces (ACMI+wall) on either side of the ACMI.