There are any number of licenses you can pick for a plugin. GPL amongst others allows other people to copy and redistribute your code under certain conditions. Most noticeably the requirement that any fork also be GPL. Other open source licenses have other requirements. If you pick the GPL license or any other open source license for your plugin and someone clones your plugin it is not a copyright violation as long as they follow the requirements. As it pertains to plugins uploaded to BukkitDev or Curse, things are slightly different. We feel the spirit of open source licenses is to allow for continuing an abandoned project, or forking and creating a new project based on the original. The purpose is not to allow anyone and everyone to create a straight 1-1 clone of actively developed plugins. On BukkitDev we will decline to host simple clones of existing plugins, regardless of licensing legalities. If you are aware of a straight 1-1 clone of an existing plugin please report it via the site and we will investigate. Forks of projects with compatible licenses will be allowed providing they are not 1-1 clones. This should represent a non-insignificant change of the code including package name changes, rewrites of existing functionality, and/or additions of functionality. Also important is that plugin names themselves are not normally licensed the same as the code. Many licenses specifically prohibit the use of the plugins or plugin author's names in your derivative. We always recommend you try to work with the existing projects to contribute directly. If you do fork an open source plugin it's always best to give attribution in your project description for the inspiration and original efforts. Ultimately, please be careful when picking your license, and if you are going to clone something, make it your own.