Able to assign health worker to two requests at the same time

[nus-se-bot]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

1. Create two request with different names but same time.
2. Assign one health worker to both request.
3. Request will be successfully assigned.

Expected behavior
As stated in the user guide 'Health worker that is assigned must be available at the current time stated by the request.'. However, we are able to assign a health worker to two different requests

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Reported by: @chanqingzhou
Severity: Low

_{[original: nus-cs2103-AY1819S2/pe-dry-run#615]}

[merody66]

Healthworker cannot cater to two requests at the same time physically, @daviddl9 pls have a look~

[jing1324]

Fixed as of PR #216