Warn when 'add item' will override an existing value?

Issue Type: <b>Bug</b> **Repro** 1. Using the `files.associations` setting 1. Add a key `abc` in the setting UI 1. Now click add item again and enter the key `abc` and a different value **Bug** This silently overwrites the previous value. Perhaps we should show a warning in this case VS Code version: Code - Insiders 1.48.0-insider (d13d2fc56da7a2f8bcad4256212db0661fcbba45, 2020-08-05T05:26:44.946Z) OS version: Darwin x64 19.6.0 <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz (16 x 2400)| |GPU Status|2d_canvas: enabled<br>flash_3d: enabled<br>flash_stage3d: enabled<br>flash_stage3d_baseline: enabled<br>gpu_compositing: enabled<br>metal: disabled_off<br>multiple_raster_threads: enabled_on<br>oop_rasterization: disabled_off<br>protected_video_decode: unavailable_off<br>rasterization: enabled<br>skia_renderer: disabled_off_ok<br>video_decode: enabled<br>viz_display_compositor: enabled_on<br>viz_hit_test_surface_layer: disabled_off_ok<br>webgl: enabled<br>webgl2: enabled| |Load (avg)|2, 2, 2| |Memory (System)|32.00GB (0.15GB free)| |Process Argv|| |Screen Reader|no| |VM|14%| </details> <!-- generated by issue reporter -->