how to "fork" forked repository?

[tansy]

Do you know how to "fork" forked repository? Like here, you have forked from jinfeihan57/p7zip and whey I want to fork your cielavenir/p7zip then I get this error (No more forks can be created. These forks already exist). Now I don't know how to contribute to your repo then.

And how did you created those links to other repositories? Like here you link fast-lzma, lizard and so on.