`extend` incorrectly allows repeating of non-repeatable directives

[m14t]

While working on #2276, it was discovered that it was possible to extend a type of a non-repeatable directive multiple times. [comment].

Simple example that should NOT be allowed:

directive @foo on SCALAR

type Query {
    someField: SomeScalar
}

scalar SomeScalar @foo

extend scalar SomeScalar @foo

It's my understanding that extend should simply try to add the directive here to the previous list of directives, and that should only be allowed if the directive is either not present, or is repeatable.

PR with failing test case: #2440

[IvanGoncharov]

@m14t Thanks for tracking this issue and especially for PR with a test case.
Fixed in #2446