Not always... For instance, the new OnCue under construction on the north side of I-240 and Sooner will compete with two other gas stations on the south side of the highway... but neither are a 7-Eleven. The nearest one of those is a mile south. Likewise, the older store at SW 59th and Western competes with no other fuel retailers at that intersection; the nearest 7-Eleven is half a mile away at SW 66th and Western.

I'm sure, though, that OnCue does something that many other retailers do - they know that a certain retailer has done their demographics homework, and if they locate nearby they're sure to pull in a satisfactory amount of business. Burger King locating near McDonald's is one example that someone in this forum has used (Pete, I believe).