Where you run into problems, is when you have Two (or more) Worlds that so good that you have to make Cores out of them, but they are in the same star system or nearby star systems, say 2 Precursor worlds of 15 and 18. Worlds like that need to be developed, which means they have to become Cores, in order to build on their surfaces.
The problem is that they will split the local colonies sending resources to them into two parts, which leads to smaller available production values for each of them, which makes it harder to build stuff at them. Particularly, they will have 2 shipyards building independent of each other, instead of one ship yard getting production from both like in GC 3.