From my point of view, competition amony friends has a positive effect on the friendship.
Why, you may wonder, should someone make friends with some specific person? It is just because they share some common interest. So it is very likely that they would have some competations constantly. However, even though these competations do exist, friends on both side can benefit from these competations. Before the competation, someone may just know another friends are good at some things, for example, the friend Tim has good command of programmaring. What he may not aware is which part do his friend Tim gifted at. After some competations between them. They may find some skills their actually never heard of, while his friends are totally a expert who knows every detail about that. So even only one person could win the competation, what important is, they know that they can learn somethings very useful from their friends, at the same time, they would treasure the friendship even more.
Furthermore, after they know from the competation about which part their friend can finish excellently, they can cooperate even better. Cooperations between friends happen more time than competations. However, some members of team just don't quite familiar with which parts are they better than others, which parts should they share the responsibility with others, and which do they need to ask for others help. All these can be clarified with a simple competitation between friends. After the competation, they can better arrange the work which best suit some specific person. For instance, to create a software, they would know which person can promote the software and which can write the code. So because of competation between ft, they can strengthen the friendship and can create a better outcome.
In conclusion, competation between friends can have a positive impact on the friendship.