I tried an experiment this morning, to make the black lines go away. As I did the first (horizontal) flip, I used the Distort/Spherize filter at -15% on the Horizontal only. For the second (vertical) flip I used Distort/Spherize filter again at -15% on the Vertical only. For the 3rd 90 degree flip I selected the layer, feathered the edge by 25 pixels, & used the eraser on the vertical edges to avoid the line. I don't know if I like it, but it was fun to try it out.
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