Add c.duplicate_declaration to warning options.

[julien-lecomte]

I have a project with many files that are similar to C syntax. Each file is compiled as if it was standalone program. For this reason, some variables are documented in what would be the equivalent of a .c file.

However, some of those variables have the same name, but not necessarily the same purpose / documentation. For this reason, I get many warnings: Duplicate C declaration, also defined at.

I would like to neither move them to a common file, nor make their documentation the same. Meanwhile, I would also like to keep the "-W" for my CI.

My current solution was creating a logging.Filter to disable the warning, but I think that an option to suppress_warnings would be beneficial to many users.