'selectionChange' event emitted twice, when selecting item that it has empty value.

[tkmknd]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

'selectionChange' event always emitted only once, when changing value.

What is the current behavior?

'selectionChange' event emitted twice, when selecting item that it has empty value.

e.g.
<mat-select (selectionChange)="changeEvent()">
<mat-option>sample</mat-option>
</mat-select>

What are the steps to reproduce?

Providing a StackBlitz reproduction is the best way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

What is the use-case or motivation for changing an existing behavior?

Since event emitted twice, event function (e.g. : changeEvent()) is executed twice.
If I would't like to execute event function twice, I would have to implement check code to prevent function from executing twice.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular:6.1.4
Angular Material:6.4.6
OS:Any
TypeScript:2.7.2
Browser:Any

Is there anything else we should know?

[crisbeto]

Closing as a duplicate of #10675.

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