Rework `get_duplicate_forecasts()` to S3 to avoid hard-coding columns and types

[nikosbosse]

          Again if we can rely on the input being a forecast object we can make this s3 and avoid a new class requiring hard coded changes to the package

_Originally posted by @seabbs in https://github.com/epiforecasts/scoringutils/pull/837#discussion_r1696782361_