I think we are comparing oranges and apples here....
Greggs is a food shop, it sells consumable goods which will get eaten by people within it's catchment area or it simply wouldn't exist. Same could be said about Starbucks/Costa/EAT/Pret etc in London, they are all within a few yards of each other.
Stores selling games and entertainment are different though.
With Game, who let's face it are the only company on the high-street specifically selling games they are as others have pointed out in a position where they bought out other companies. Where I grew up, I had a summer job in a Game in Hemel Hempstead some 11 years ago now, there was at one stage a Game, EB and Gamestation within yards of each other. They eventually rebranded the EB to Game before getting rid of it a few years later.
I think though that this year will see a lot of duplicate shops disappearing, what with changes in spending habits, the VAT rise and more people buying online. HMV has already announced it's not doing well and that they are actually closing their second Oxford Street store... that's pretty major when you think that it's Europe's busiest shopping high-street. So I'd imagine we will see many shops going this coming year.