Can someone apply with the same design after my protection is over? How?

A U.S. Patent is a contract between the government and the inventor. The government grants to the inventor a limited monopoly, that is, the right to exclude all others from making, using, or selling the invention within the U.S. during the patent term. In exchange, the inventor gives the public a full and complete disclosure of the invention with a teaching of how the invention works. Once the term of the Patent expires, the invention enters the public domain and can be used by anyone. Another inventor may obtain a Patent on an improved version of your Invention. A Design Patent will protect the design for 14 years from the date that the Patent Issues. A Utility Patent will protect the invention for up to 20 years from the date that the Patent Application is filed. Depending upon the nature of your invention, A Design Patent or a Utility Patent may be preferred, or you may be entitled to both. If you do not file a Patent Application for your Invention, someone else may, and you may be barred from practicing your own Invention. I hope this helps and Good Luck with your Invention.

Since your patent will be prior art, as well as because you were the original inventor, the subsequent patent application should be rejected because a that patent would be invalid.

No. After your period of protection ends, the design is available to everyone unless the design has somehow come to represent a particular product to the general public. For example, the classic design of a glass Coca-Cola bottle now serves as a trademark for the product.

They can apply and may even have it granted, but it would be invalid.

The short answer is once your design patent expires, you have no patent protection on the design, but that is not the end of the story. It is possible that you have acquired trademark and or copyright protection through your use. You would need to talk with a patent/trademark professional to be sure.

NO - once your design patent expires - the design enters the public domain - and all can use it freely. However - your patent also becomes "prior art" to anyone else - preventing the patenting of the exact design again. A modified version might be patentable - if it is more than an "obvious" modification of your design.