Objects, squares duplicated one to each other have a visible hairline inbetween

hello everybody,

I hope its clear what I mean. Although I select to duplicate s.th. exactly one to the other I get this annoying effect. For example when duplicating a 100mm black square next to the other I always get that minimal small hairline which is visible on screen and really there between both objects. When I print that on 720dpi I have a small white hairline between. With 1440dpi I can avoid it. probably because of more ink that expands a little.

i would be happy if you have an idea that helps. Many thanks

Uli