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I don't know how D. mercatorum is supposed to look like.
According to Bächli's key, there shouldn't be a dark costal spot as in D. repleta - but the size of the light lateral spots and the thin distal stripes on the tergites (>1/2 width in D. repleta vs. ~1/3 in D. mercatorum) raised my suspicion here.

Additionally, Jan Maca once mentioned in a comment that the presence of the costal spot should not be overrated.

So, here's my fly to be discussed.