Publishing independent proofs of a theorem in the same journal?
I can only speak for mathematics. However, when you really understand the nature of mathematics you start to realize that the proofs of the theorems are often as or more important than the statements of the theorems themselves. 

The proof, in showing why something is true, gives a roadmap to truth. Sometimes a technique, if it is not standard but is interesting, can be the most important aspect of a math paper because that same roadmap might just show a way to prove other things, some similar, and some not. 

My own dissertation had interesting theorems, to be sure, but it was most useful for the proof of one of the theorems. The proof was unexpected and gave new ways to approach some problems in Analysis. 

So, a journal that shows two independent proofs of the same thing can be especially interesting since the similarities and differences between the proofs can give hints of other things that might be shown. 

I doubt that it is especially common, though in popular areas of research it must happen. Parallel research is pretty widespread, though if offset in time by only a bit, it won't be possible to have such things in the same issue of a journal. 

On the other hand, getting beaten to a result may not be devastating if different approaches are taken. Other mathematicians can learn from that. 

From the journal's standpoint such situations are especially satisfying as two different but interesting proof methodologies may represent the merging of two separate trains of thought. That in itself is interesting to a mathematician and may lead to consolidation or to further advances.