fix(router): RouterOutlet loads component twice in a race condition

[mhevery]

• Please check if the PR fulfills these requirements
• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

bug fix #7497

• What is the current behavior? (You can also link to an open issue here)

During race condition it is possible that the routerOutlet will activate a component multiple times.

• What is the new behavior (if this is a feature change)?

Ensure that activation can happen only once.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Other information:

[mhevery]

@tbosch this is a fix for #7497. I need help writing the test which demonstrates the issue.

[mhevery]

@tbosch LGTM, per our discussion

[mary-poppins]

Merging PR #7545 on behalf of @jelbourn to branch presubmit-jelbourn-pr-7545.

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