Fix potential hang when duplicated task registered.

[windtalker]

What problem does this PR solve?

Issue Number: close #8192

Problem Summary:

What is changed and how it works?

Only unregister and remove from mpp task monitor if the task is registered successfully.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[yibin87]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: SeaRise, yibin87

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-10-13 08:30:47.855560457 +0000 UTC m=+1386645.442670601: ☑️ agreed by yibin87.
• 2023-10-13 09:41:57.087209912 +0000 UTC m=+1390914.674320057: ☑️ agreed by SeaRise.

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #8195.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #8196.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #8197.