Okay so here's an image with the problem
If I change the border color to white, I obtain an image similar to the one when duplicating the border pixels(see below), without the black pixels in the corners.
Here's the result after shifting the texture coordinates, that means if for example the initial coordinates are (0,0), (1,0), (1,1), (0,1), after the shift they will become (0+shift, 0+shift),   (1-shift, 0+shift), (1-shift, 1-shift), (0+shift, 1-shift). The effect is that the lightmap is stretched, but the seam monster is still there:
Note that when duplicating pixels I'm not changing the pixels in the corner of the lightmap(they remain black, well I left them like this because there was no need to calculate their value since this method does not eliminate the seams). To clear things up: by duplicating I mean: copy collon 0 of the lightmap one pixel left, copy line 0 of the pixels one pixel up, and so on.
Here's what I get if I calculate light intensity for the border pixels, it's perfect :)
Best regards,

Bored stiff? Loosen up...
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