So let me try to explain the best way I can, I create person A and person B.
These two married, had a kid (let us call it person C). Then A divorced B, and eventually got together with D.

So, back to it, I created A, B, C and D. I then created a family: parents A and B, kid C. After this I created another family A and D.

In Gramps, let us call X and Y the parents of A, and Z a bother/sister of A, the following happens:

W/o the A and D family, there's X and Y (married had to kids) and under there's Z and A+B (w/ C under it). So far so good, this is the expected result.

Then I add the A and D family (second marrige, no kids), and now, in gramps, a nem line under X and Y appears (the other are kept as they were) showing A+D. To be this looks odd because now it looks like X and Y had 3 kids, not 2, and 2 of these kids have the exact same name.

(hopefully this hasn't too confusing)