Justify another citation

Suppose you have one citation that states something then the second source gives you additional illustration, but it doesn't necessarily establish a substantially new proposition.

So it could become unnecessarily repetitive

The phrase “justify another citation” means:

Is the additional source contributing a sufficiently new, important, or independent piece of evidence to make adding it worthwhile?

If yes, cite it.

If it merely repeats what your scholarly source already establishes, you don't necessarily need it.