OnPaint event is always called twice - c++builder

Question

I have a simple implementation for the OnPaint event in my Borland C++ (Delphi) program. In OnPaint in an MDI child, a grid is drawn and inside the grid I show a number of bitmaps.
When I open/create a new Window, this is what happens:
The window is shown.
There is an OnPaint event, in which I draw the grid and the bitmaps, but some of the bitmaps do not show completely, there are some lines in the window where the part of the bitmap is not shown.
Then there is a next OnPaint event, the same code is called and now all the bitmaps are drawn completely.
Other users of the program report more than 2 OnPaint events, with similar failures in the bitmaps.
I included a counter, the counter shows that OnPaint is, in my case, always called twice when I create a new window, when the window is redrawn as result of moving, OnPaint is only called once. I disabled some of the drawing code to see if something here is causing the second call, but this is not the case.
Is there an explanation for this behavour, and better, a cure ?
void __fastcall TfrmSpoor::FormPaint(TObject *Sender)
{
... do some calculations
HorzScrollBar->Increment = PixelsPerHokX;
HorzScrollBar->Range = PixelsPerHokX * Railplan->xGrootte;
VertScrollBar->Increment = PixelsPerHokY;
VertScrollBar->Range = PixelsPerHokY * Railplan->yGrootte;
CanvasOffsetX = (int)-HorzScrollBar->Position % PixelsPerHokX;
CanvasOffsetY = (int)-VertScrollBar->Position % PixelsPerHokY;
Canvas->Lock(); // reserveert de Canvas voor deze thread
OnPaintTeller++;
Color = temp.GeefSymboolKleur(Vormgeving->SymboolGrootte);
... do some more calculations
{
TekenRaster(Canvas, Deelplan);
TekenRailplan(Canvas, Deelplan, true);
}
// Set some Buttons to visible:
ToonESTWRegels(Vormgeving->SymboolGrootte == SG_ESTW && Vormgeving->ESTWButtons);
Canvas->Unlock(); // release Canvas
}