Tasks not respecting instance limit

[Novynn]

Issue Type: Bug

VS Code version: Code 1.44.2 (ff91584, 2020-04-16T16:36:23.138Z)
OS version: Windows_NT x64 10.0.19041

Repro:
Using the following tasks.json:

{
    // See https://go.microsoft.com/fwlink/?LinkId=733558
    // for the documentation about the tasks.json format
    "version": "2.0.0",
    "tasks": [
        {
            "label": "run update.sh",
            "type": "shell",
            "command": "./update.sh",
            "options": {
                "shell": {
                    "executable": "bash.exe",
                    "args": [
                        "-c"
                    ]
                }
            },
            "group": {
                "kind": "build",
                "isDefault": true
            }
        }
    ]
}

Observed Behaviour:
Holding down Ctrl+Shift+B (run default build task) will result in multiple tasks running at the same time. This clogs up the "currently running tasks" list especially when a task hangs.

Expected Behaviour:
A notification should tell us that the task is already running. This is what I see using version 1.42.1.

System Info

Item	Value
CPUs	Intel(R) Core(TM) i5-6600K CPU @ 3.50GHz (4 x 3504)
GPU Status	2d_canvas: enabled flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on oop_rasterization: disabled_off protected_video_decode: unavailable_off rasterization: enabled skia_renderer: disabled_off_ok video_decode: enabled viz_display_compositor: enabled_on viz_hit_test_surface_layer: disabled_off_ok webgl: enabled webgl2: enabled

[vscodebot]

(Experimental duplicate detection)
Thanks for submitting this issue. Please also check if it is already covered by an existing one, like:

• default run task (#70118)
• Task: Strange task name in recently used section (#93934)
• Shortcut Key For Task Shows Task List (#94654)
• Improve Run \[Build\] Tasks command and dialog (#47449)
• Task: removed tasks still available in "recently used" (#94020)

[bmartin]

I've been seeing this too. This causes corrupted builds in my project since I'm in the habit of invoking builds frequently and the build is not safe to running in parallel with itself.

[alexr00]

Thank you for reporting this! Duplicate of #96999