Fail early and clearly on fasta files with duplicate IDs

[Alisandra Denton]

Currently duplicate IDs (after splitting at " " character) cause an error only after pre-processing & NN prediction have completed successfully (i.e. after substantial wall time) and HelixerPost started, resulting in an error of the form

thread 'main' panicked at 'Failed to open input files: Duplicate Value: Block Start 0 at index 2 already occurred at index 0', helixer_post_bin/src/main.rs:30:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

An error occurred during post processing. Exiting

This should be checked at import, and have a clearer error