Release version 1.1.19 - Enhanced duplicate detection

Fixed persistent duplicate settings page by improving duplicate detection
to check by class type and ID instead of strict instance comparison.

Issue:
- Strict instance comparison (===) failed to detect duplicates
- Different object instances of same class not caught by previous check
- Settings page still appearing twice despite singleton and instance caching

Solution:
- Changed duplicate detection from instance comparison to type/ID checks
- Added instanceof WC_TPP_Settings check
- Added get_id() method check for ID 'tier_package_prices'
- Added direct id property check as fallback
- Multiple layers of detection to catch duplicates regardless of how created

Fixes:
- Settings page rendering twice in WooCommerce backend
- Duplicate detection failing for different instances of same class

Changes:
- Enhanced add_settings_page() with multi-layer duplicate detection
- Checks: instanceof, get_id(), and id property
- Returns early if any existing page matches criteria

Technical Details:
- Strict comparison only works if exact same object instance
- Different instances (even of same class/ID) fail === check
- instanceof checks class type regardless of which instance
- ID checks ensure no duplicate pages with same identifier
- Fallback checks handle different WooCommerce versions/implementations

Updated Files:
- includes/class-wc-tpp-admin.php (enhanced duplicate detection)
- wc-tier-and-package-prices.php (version 1.1.19)
- composer.json (version 1.1.19)
- CHANGELOG.md (v1.1.19 section)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

composer.json

@@ -1,7 +1,7 @@
 {
     "name": "magdev/wc-tier-package-prices",
     "description": "WooCommerce plugin for tier pricing and package prices with Twig templates",
-    "version": "1.1.18",
+    "version": "1.1.19",
     "type": "wordpress-plugin",
     "license": "GPL-2.0-or-later",
     "authors": [