@std/ini parse silently drops an entire section when the section name repeats #7360

Describe the bug

When an INI document contains the same section header twice, parse keeps only the last occurrence's keys. The earlier section's keys vanish without an error, so a valid config loses settings silently.

Reproduction

import { parse } from "jsr:@std/ini";

parse("[s]\na=1\n[s]\nb=2");
// -> { s: { b: 2 } }   (a=1 is gone)

parse("[db]\nhost=a\nport=1\n[db]\nuser=b");
// -> { db: { user: "b" } }   (host and port are gone)

Expected behavior

Other mainstream INI parsers do not lose the earlier content:

• node-ini merges repeated sections: { s: { a: "1", b: "2" } }
• Python's configparser raises DuplicateSectionError (in strict mode, the default)

Either merging or raising protects the caller; silently discarding the first section does not. Repeated sections can appear when config files are concatenated or appended to, so this is reachable in practice, not just hand-written edge cases.

Environment

• @std/ini (JSR, latest)
• Node 22 for the harness; the code path is engine-independent