Bitmap region is already locked

Bitmap b = (Bitmap)pictureBox1.Image;
            Bitmap b2 = (Bitmap)pictureBox1.Image;

            // GDI+ still lies to us - the return format is BGR, NOT RGB.
            BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
            BitmapData bmData2 = b2.LockBits(new Rectangle(b.Width, b.Height, b2.Width, b2.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

            int stride = bmData.Stride;
            int stride2 = bmData2.Stride;

			unsafe
			{
                byte* p = (byte*)bmData.Scan0;
                byte* p2 = (byte*)bmData2.Scan0;

                int nOffset,nOffSet2;
                int nWidth,nWidth2;

                NOffSet(b, stride, out nOffset, out nWidth);
                NOffSet(b2, stride2, out nOffSet2, out nWidth2);
	
				for(int y=0;y<b.Height;++y)
				{
                    for (int x = 0; x < nWidth; ++x)
                    {
                        if (p[0].Equals(p2[0]))
                            listBox1.Items.Add(string.Format("({0},{1}) :{2}", x, y, p2[0]));
                        ++p; ++p2;

                    }
					p += nOffset;
				}
			}

			b.UnlockBits(bmData);