How can I remove the horizontal lines while I'm playing?

[Microsoft Windows]

I have two monitors connected to my PC, the main one connected to DVI and the other connected to HDMI. When I play some games on the main screen there are always horizontal lines that can be removed by activating v-sync, but this decreases the fps. On the second screen instead, the game doesn't have horizontal lines and there is no need to activate v-sync, so I can play more fluidly. What can I do to remove horizontal lines from the main screen too?

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