fix(watchThrottled): duplicate callback invokation when leading and trailing edges coincide

[bodograumann]

• chore: remove duplicate test
• fix(throttleFilter): only invoke once when leading is also trailing edge

Description

When using watchThrottled and only exactly one change occurs in the source, the callback is still called two times: once immediately on the first change and once after the timeout.
This second call should not occur, because the leading and trailing edges actually coincide, when there is a single event only.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
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• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
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