My Simplex + uses the 2.77 update, and in trashier areas I use Park 1, and low-trash open areas eight Field or Beach.
I always start at maximum Sensitivity and reduce it when I happen to encounter EMI. I only have the first Disc. segment rejected in the Field mode, Park 1 and Beach have zero Disc. notches rejected, and I never use Park 2.
When I get EMI the Sensitivity is reduced, slowly, just to the point it runs stable. I work the coil at a proper ±2" coil-to-ground position and that helps eliminate multiple hits from closely-positioned coins and such. Due to the circuitry design, power level, and the use of a Double-D coil, it is easy to get multiple hits from targets too close to the search coil's inner EMF so proper coil height is important.
I use the 5X9½ DD mid-size coil most of the time and seldom experience any issues. As for one-way hits they can happen with any search coil type, but seem to a little more often with a DD coil. It can also be caused by a canted target or odd target shape and orientation to the coil. Another reason is a nearby trash target that causes some rejection and the unit can't recover in time to respond to the favorable target, especially based on the sweep direction and sweep speed. A good audible 'hit' from the left or from the right, still needs to be recovered to know if it's a good or bad target.
Monte