verify on deduplication

[milyin]

multiple execution of proc-macro attribute may cause multiple json files with same content in outdir, this is normal.
But hypothetically it's possible that different jsons contains different definitions for same type. This may happen due to user error: using prebindgen attribute on objects of same name without grouping.
This may also happen when code is modified and outdir is not changed (rust-analyzer?)
In any case this should be diagnosed and cause build failure