Why is it considered sub-par to re-test something that has been shown before as opposed to addressing a knowledge gap? Firstly, apologies if this is the incorrect StackExchange child to post this one. The only other study design questions I could find were here.

So...

Common feedback I get as I plan different projects for my PhD are "explain where the research gap is" or similar.

My question is: shouldn't it be encouraged to re-test something that has been shown before?

For instance my field is in ecology. So if someone elsewhere in the world has shown an effect on X on Y in a different ecosystem, it makes sense to me to test if it occurs in the ones near me. In some professions, e.g. clinical trials in medicine, completing the same experiments is common.

Or, am I misinterpreting this and I could in fact make the argument the gap being there is no local studies? Nonetheless, if I did this, I feel it is frowned upon to imply that the outcome of the study may already be known. But, this attitude I feel discourages re-testing ideas. 

After all, if there is such a massive focus on making science reproducible, but no one ever reproduces it for fear of not doing a novel experiment, then what is the point in making science reproducible?
In short, a PhD is about something that has not been done before. If there is no gap in the literature, there is no PhD, and defining the lack of knowledge is the first step towards covering it. A PhD is about novelty and exploration, not tweaking, and even PhDs that improve existing methodologies need to fulfil that requirement in some way by showing theoretical or technical expansions, or at the very least an application from field B that is novel in field A (even that for many people is not enough). A PhD explores ground already covered is pointless because it does not show creativity and originality of thought, which are fundamental for a researcher.

On the other hand, academic papers can (and frankly should much more) deal with reproducibility and testing/verifying existing results. There is a strong tendency in modern research to present positive results and novel findings, so papers that deal with what has not been found or not has not been replicated fall out of favour. That is an important problem, in my opinion, and should have been encouraged by the practices of conferences and journal editors/ referees, but belongs to a different discussion (even commentaries are under extinction).