Transforms outputting to same file silently overwrite each other

[alterae]

Minimal reproduction:

.
├── layout
├── mksite.toml
├── src
│  ├── foo.1
│  └── foo.2
└── static

# mksite.toml
[transforms]
1.txt = "cat"
2.txt = "cat"

This will result in only one page called foo.txt in out/.

This behavior should probably be at least a warning, probably an error.