There is nothing wrong with creating an Area51 proposal that is similar to an existing proposal. I personally don't think that the proposal "Metallic Cartridge Reloading" will succeed, Warren has made some good arguments (IMHO) on why the "Weapons" proposal has a better chance to succeed.
The creator user1801810 disagrees, and that is fine. We all get our own opinions, and we can all choose to support whichever proposal(s) we like best. Which is why proposals (mostly) don't dupe against each other on Area51. You can follow all of them, and even post the same questions on all the related proposals.
There are milestones that proposals must reach, if they fail to meet them, the proposal is closed.
If the proposal Metallic Cartridge Reloading" hits the milestones and continues to move forward, that is fantastic, I wish it the best.
If not, supporters may want to consider one or both of the other existing proposals.