fix: do not send duplicated events in batch

[Ian2012]

Description: Describe in a couple of sentences what this PR adds

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[openedx-webhooks]

Thanks for the pull request, @Ian2012!

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[bmtcril]

Looks good, can you add a test that triggers the duplicate logic?

[bmtcril]

Can you comment why you're doing this? I'm sure I will forget as soon as this is merged. :)