Ask YC: Will YC compete with YC?
1- direct competition with one of its current companies and/or products
2- possible future competition of one of its current companies and/or products
3- if the new proposed idea is not really in competition with any of YC's companies, but it does have some major ideas, that another YC company could take advantage from, or add in as new feature to their system?
of course, if there is already competition then your idea might not be as interesting, regardless of whether the competition is a YC company or any other company. I guess the real question is whether they review any new application without considering what they previously own and give it a fair treatment or not?