duplicate definitions warning

[hiteshgupta9193]

People can define the same message with the same id and same default message in different files. Although it will generate single output in the output file, some people may want to give developers a warning in the development mode.
Please add a flag to give these types of warnings or some functionality to run a method over every message item to give users control to write their own logic before generating the final output.
Can be done something here - https://github.com/formatjs/formatjs/blob/main/packages/cli/src/extract.ts#L212