This happens with mostly every asset, not just decals. Shirts and pants have their textures, as do hats and T-Shirts. Decals do it. Unions do it. Meshes do it four times (?); two for the mesh and two for the texture. Animations do this… I think?
You have the item in which you can possess and one for which the asset references when you plug in its Id. If you notice when you paste an Id from your inventory of an asset, the numbers beside “rbxassetid://num” are different from the numbers you pasted (https://www.roblox.com/catalogOrLibrary/assetid/name).