We are lucky to be part of a fandom where there is nearly always an event happening that encourages the creation of fanworks (as Around the World and Web will attest). When the SWG was formed, we decided as part of our mission that we wanted our group to behave in ways that support other fandom projects and events to the greatest extent possible, rather than viewing other events and sites as competition.
As a result, we not only allow but encourage challenge responses that meet not just our challenge but that fit other fandom events and challenges as well. (Of course, check that this is permissible for the other event. We can't speak to their rules, just our own!)
Some events and challenges may require you to post with them first. That is fine; challenge responses don't need to be posted first with us in order to count. (They do need to be posted to our archive, however.)
If an event has a reveal date after our challenge deadline, or if you are not allowed to crosspost until after the challenge deadline, you can still receive the stamp for the challenge. If possible, contact the mods ahead of time to give us a heads up, and when the fanwork has been posted to our archive, let the mods know you need the stamp, and we will add it.