I understand the requirement for avoiding duplicate links, but there should probably be a way to make an exception for the main website of the project (maybe enforce the rule on all links except for one, which can show up multiple times?).

The reason is that the project's website link will probably show up in the main description of the list and then also in the list itself as one of the core links where people should go right in the beginning. It can also be set as a link from the header image.