What is the issue?

When assembling an object via list comprehension, duplicated files are allowed when they should not - see the same example running on sjsonnet.org

How to reproduce?

1. echo '{ ["bar"]: x for x in [37, -987]}' > foo.jsonnet
2. ./mill "sjsonnet[2.13.3]".jvm.run foo.jsonnet
3. Expected:

RUNTIME ERROR: duplicate field name: "bar"
	example1.jsonnet:1:1-35	

4. Actual

{
   "bar": -987
}