When i duplicate the layer two or three times, one above the other (using alpha images) the image looks better, and brighter, specially when the png alpha have lines (for example, in a image of the solar system, the orbits does look a lot better if i duplicate the images) but is there any way, maybe an effect, to virtually duplicate the layer without having to actually duplicating it? I'm trying to do this in order to save render time, but also, does it really affect the render time, having three layers, as example, on the same render, duplicated?
Except for the actual blending calculations across the layers it doesn't have much of an impact. The rest would require to see the footage, but presumably you should be able to achieve similar results with Levels adjustments on the Alpha channel and some boosting on the color channel as well. In fact this may simply be due to your transparency having a Gamma bias in the first place which can easily be tweaked and corrected this way.