Check fields duplication

[tetianakravchenko]

It is possible to define the same field multiple times in <package-name>/fields/ folder. If the definition of this fields are different - it might cause unexpected changed wen runnin elastic-package build, example: elastic/integrations#6118 (comment)

I think it would be helpful to add check on duplicated fields when executing elastic-package check

[jsoriano]

This check (along some others), is already available in package spec v2 (elastic/package-spec#331). Try updating format_spec in the manifest to a recent version, like format_version: 2.8.0.

If you find issues upgrading to this major, take a look to these guidelines: https://github.com/elastic/elastic-package/blob/main/docs/howto/update_major_package_spec.md