fix: preserve repeated group roles in VS Code Compare #52

Follow-up to PR #50 and the review finding.

The storage format permits the same record in multiple roles within a group. set_compare([{record: 0, label: "reactant"}, {record: 0, label: "product"}]) produces duplicate record IDs and overwrites one label. Compare keys its viewer list by record ID, so opening this valid group can throw a duplicate-key error.

Preserve membership identity, or deliberately deduplicate and retain all role labels. Apply the MAX_COMPARE limit after the chosen policy. Add a regression case with the same record in two roles.

Acceptance: Compare renders the group without throwing, and users can see both roles without silently losing a label.

Explicitly deferred from PR #50 at the user's request; avoid sending groups with repeated members to Compare until fixed.