I would describe both of these polishes as a purple jelly base with irregular duochome flakies, tiny blue shimmer and random pieces of opalescent blue glitter.
I swatches these both side by side to show you just how they compare to one another. The photo below shows 1 coat, left to right Orly and OPI.
At one coat I could see a difference right away, the base of FP is more blue than MM, which is more red. It is a slight difference, but noticeable to me I'm so glad it shows in the photo! Sometimes when I do comparisons the differences are more apparent in person.
Other than the slight base colour difference, I also noticed that the flakie size differed. MM has larger chunks where as FP has smaller flecks.
Here they both are again at two coats...
And three coats.
Different angle from the light to see the base shade better. Top to bottom, Orly, OPI
Left to right, OPI, Orly
Natural sunlight, left to right, OPI, Orly
These differ slightly from one another so I wouldn't say that they are dead on dupes. However, if you couldn't get your hands on Merry Midnight when it was available, Fowl Play is a great substitute!