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The author of a work may use a license to permit other people to use the work in specific ways, subject to author-defined license-specific rules. Allowing other people to use the work via a license in no way lessens the author's copyright -- on the contrary, the author's copyright is what makes the license possible and necessary.  (One exception to this is if you offer an exclusive license to some particular person or party, including a contractual promise not to offer additional licenses, in which case you may well limit your ability to reproduce your own work.)

The author of a work may use a license to permit other people to use the work in specific ways, subject to author-defined license-specific rules. Allowing other people to use the work via a license in no way lessens the author's copyright -- on the contrary, the author's copyright is what makes the license possible and necessary. 

(One exception to this is if you offer an exclusive license to some particular person or party, meaning that you make a contractual promise not to offer licenses to anyone else, nor to exercise your copyright rights yourself. In that case you do limit your ability to reproduce your own work, but this case is quite different from offering a non-exclusive license to the public at large.)

The author of a work may use a license to permit other people to use the work in specific ways, subject to author-defined license-specific rules. Allowing other people to use the work via a license in no way lessens the author's copyright -- on the contrary, the author's copyright is what makes the license possible and necessary.

The author of a work may use a license to permit other people to use the work in specific ways, subject to author-defined license-specific rules. Allowing other people to use the work via a license in no way lessens the author's copyright -- on the contrary, the author's copyright is what makes the license possible and necessary. (One exception to this is if you offer an exclusive license to some particular person or party, including a contractual promise not to offer additional licenses, in which case you may well limit your ability to reproduce your own work.)

If someone takes your GPL-licensed project and distributes a fork with their own changes without interacting with you (which they can do as long as their changes are under a GPL-compatible license), then you are free to incorporate their changes in your own project. However, if you do so, then you are no longer the sole author of the work, and you must abide by the other author's license as long as you use the material they wrote.