Problem

The IDL parser checks for duplicate feature flag names only within a single file. Cross-file duplicates are silently accepted at parse time and surface only as a runtime uassert (code 23784) at server startup. The Python tooling that processes IDL file silently overwrites earlier definitions on a duplicate name.

A developer can declare the same feature flag name in two different IDL files and receive no compile-time or generation-time error. The failure mode is either a server crash at startup or, in the Python tooling path, silent data loss (the second definition replaces the first).

Steps to Reproduce

1. Declare a feature flag with a given name in query_feature_flags.idl (or any IDL file).
2. Declare a second feature flag with the same name in a different IDL file (like sharding_feature_flags.idl).
3. Build and run mongod — uassert 23784 fires at startup.
4. Run the Python IDL tooling — the second definition silently overwrites the first with no error or warning.

Expected Behavior

The IDL code-generation pass should detect duplicate feature flag names across all input files and emit a fatal error — with both conflicting file names and line numbers — before producing any generated output.

Suggested Fix

In the IDL code-generation pass that aggregates feature flags across files, add a cross-file uniqueness check. On collision, report the conflicting definitions (file, line, name) and abort generation.

Impact

Developer-experience / correctness. A duplicate flag name is always a bug. Catching it at generation time (before the binary is built) is strictly better than a runtime crash or silent overwrite. This added complexity to SERVER-125794.