fix: scope duplicate key tracking per section with bracketedArray=false

[abhu85]

Summary

Fixes #298

When using bracketedArray: false, the duplicate key counter was global across all sections, causing keys with the same name in different sections to be incorrectly converted to arrays.

The Problem

[section_1]
var = 1

[section_2]
var = 2

Was incorrectly parsed as:

{ section_1: { var: '1' }, section_2: { var: ['2'] } }  // ❌ var is wrongly an array

Because the global duplicates counter saw var twice across both sections.

The Fix

Reset the duplicates counter when entering a new section (line 128), so keys are only treated as duplicates when they appear multiple times within the same section.

Now correctly parses as:

{ section_1: { var: '1' }, section_2: { var: '2' } }  // ✅ both are scalars

Changes

• lib/ini.js: Change duplicates from const to let, reset to {} when entering new section
• test/duplicate-properties.js: Add regression tests for cross-section and within-section duplicates

Test Results

All tests pass with 100% coverage.

---

[rquadling]

Purely as an "is it allowed"...

[section_1]
var = 1

[section_2]
var = 2

[section_1]
var = 3

I can't remember the rules on a section being reused in the file, If so, section_1.var either is an array or overwritten, depending upon POV.

[abhu85]

@rquadling - Great question! I tested this exact scenario. With a repeated section:

[section_1]
var = 1

[section_2]
var = 2

[section_1]
var = 3

With this fix (bracketedArray: false): section_1.var = "3" (last value wins), section_2.var = "2" — consistent with how the default bracketedArray: true mode handles repeated sections.

On main today (bracketedArray: false): section_1.var = ["1", "3"] and section_2.var = ["2"] — the section_2 single-element array is the bug this PR fixes (#298), and the section_1 array is a side effect of the same global counter bug rather than intentional behavior.

So this fix makes bracketedArray: false consistent with the default mode for repeated sections: last value wins. If someone needs array merging across repeated section headers, that would be a separate feature since neither mode supports it today.