It depends where you live. For example, Keene NH has 6 stores that least have several full length grocery isles (Walmart, Target, Dollar Tree, Price Chopper, and Hannaford's) another 6 local food markets that don't sell gas. Then there are 4 gas stations with markets in them. And less than 1 mile south there is another grocery store (Market Basket) and another gas station with a market. Plus there are farms and even a large co-op that's locally owned. Not to mention the farmer's market. There are 3 drug stores that have isle after isle of food, including frozen and things like eggs and milk. In fact, if you do coupon stacking, drug stores tend to have the best deals on name brand items, as you can often get stuff for free. Then there's the all local store and the old country store. So people in Keene have at least 25 food stores to buy groceries within bike riding distance. There was another local chain grocery store, Shaw's, but it had to close because it wasn't competitive with the near-by much smaller chain, Market Basket, not to mention the Walmart that shared a building with it. And several years back, Kmart closed because it was poorly run. So there is a good flow on companies competing in the Keene market. There are also a dozen bakeries that sell things like cupcakes, cakes, donuts, muffins... in bulk, which compete with grocery stores for those things. Then in you drive 25 minutes in 3 different directions, there are other grocery stores, food markets, gas stations with markets, dollar stores and so on.
New stores are continuously being built (ALDI, Family Dollar, Dollar Tree, Ocean State Job Lot, Walmart, Market Basket, Trader Joe's Whole Foods, Fresh Market, Ali Baba, local co-ops) and various poorly run stores continue to fail in NH (the Costco in Portsmouth, all of the Stop N Shops in NH, some of the Shaw's). The market certainly isn't a oligopoly in NH, but maybe in your community in ME, it is. That might not just be able lack of a free market in ME, considering how few people live in the area and it's in ME, which is on the edge of the US. My guess is, if ME wasn't so anti-business and anti-consumer, you might have a little more competition there, but it's hard to compete when the market is so small and there is NH right next door.