[composer] duplicate vendor

I state that my question is not mainly about php, but composer.

Here's the question ...

+ cat .config/composer/config.json
{
     "repositories": {
         "b": {
             "type": "path",
             "url": "$PWD/b"
         },
         "c": {
             "type": "path",
             "url": "$PWD/c"
         }
     }
}

+ cat a/composer.json
{
     "require": {
         "test/b": "^1.0"
     }
}

+ cat b/composer.json
{
     "name": "test/b",
     "version": "1",
     "require": {
        "test/c": "^1.0"
     }
}

+ cat c/composer.json
{
     "name": "test/c",
     "version": "1"
}

As you can see, only the *test/b* package requires the *test/c* package.
However, as you will see below, the *vendor/test/c* directory I find it 
both under directory *b*, and under the directory *a*.

+ composer update --working-dir=b
+ composer update --working-dir=a
+ find | grep vendor/test/c
./b/vendor/test/c
./a/vendor/test/c

Can the directory *a* contains *vendor/test/c* be prevented?

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