Same-same, but different.
Until as recently as 2020, Mountain Dew contained Brominated Vegetable Oil (BVO), which is banned in Europe and 100 other countries. The chemical is a flame retardant and can harm the body in many ways. Walmart’s Orangette Orange Soda still contains this ingredient and you’d want to check the labels of other sodas you pick up at the supermarket to avoid it.
There are many other snacks, treats, and foods Americans enjoy that have harmful chemicals banned elsewhere. European countries are particularly strict about what consumers are putting in their body, so the original treats Americans enjoy aren’t available, or they are manufactured without those ingredients. Next time you travel to Europe, pop into a grocery store and make comparisons.
Marabou only became a company in 1949, eight years after M&M's were introduced in 1941. Their M peanuts were first sold a couple decades later in the '60s. So if anyone stole from the other, it was M Peanut from the Mars company. And they didn't even attempt to disguise the fact, rather using a name almost the same. Sort of like what Russia is doing now with McDonalds and Ikea imitations since after their genuine founding conglomerates pulled out.