Plugin skills loaded twice from cache/ and marketplaces/ directories

Description

Skills from a custom plugin marketplace are registered twice in the context window — once from ~/.claude/plugins/cache/ and once from ~/.claude/plugins/marketplaces/. This wastes context tokens on duplicates.

Steps to Reproduce

1. Create a custom local plugin marketplace (e.g., seo) with skills (e.g., ai-humanizer, keyword-clusterer)
2. Install the plugin — it gets cached in ~/.claude/plugins/cache/seo/seo/1.0.0/skills/
3. The original source remains in ~/.claude/plugins/marketplaces/seo/skills/
4. Run /context to check loaded skills

Expected Behavior

Each skill should be loaded once — either from cache/ or from marketplaces/, not both.

Actual Behavior

Skills appear twice in /context output under different namespace prefixes (e.g., seo:ai-humanizer and anthropic-skills:ai-humanizer), consuming extra context tokens.

Example from /context:

| ai-humanizer | Plugin | 159 |
| keyword-clusterer | Plugin | 175 |
...
| ai-humanizer | Plugin | 156 |
| keyword-clusterer | Plugin | 172 |

Environment

• Claude Code version: 2.1.75
• OS: macOS (Darwin 25.3.0)
• Node: v25.7.0