Played a couple of 5-player games.

In both games 2 or more pkayers started with their factories on the same resource type. With the result that they produced 4 of their initial resources during the first round, When the other players were each producing only 2. In both games one of those players, eventually won.

Do other players find this setup advantage a serious advantage ?

We might consider a house rule to prevent it. (e.g. force all players to start with different resources)

I've never known this to be an issue in our games.  By which I mean I don't think it's happened, or if it did it certainly didn't impact the game enough for us to notice.  I'm not entirely convinced that having two players start with the same resource is a game breaking issue, but if it really bothers you it seems easy enough to take one of each planet (except Alien) and randomly deal out a number equal to the players around Sol to ensure everyone has a unique starting resource.

I've had this happen to me, and changed my game strategies to deal with it. The obvious counter-strategy is sending your starting ship to a planet of the same resource type as the majority of your opponents homeworlds. I've found this type of mutual production can happen in the middle of the game, if your opponents colonize the same kinds of planets, so I make sure I have the same kinds of planets as my opponents to take advantage of their resource production throughout the game.