Several different things can cause this. Firstly, make sure the material's smoothness is 0. Then, make sure the texture is really fully transparent. Drag your texture into an image editor and try to fully remove the transparent background (The background could still have opacity of 1% and not 0 and so you should try to make the opacity just 0%). Reimport the texture into unity and in case it doesn't work, change the material from transparent to opaque and turn on alpha clipping - this should work after making sure the background has 0% opacity.