Event Cart: honor the allow_same_participant_emails setting

[mlutfy]

Overview

When using the Event Cart, the Checkout process does not honor the "allow participants with the same email address".

This copies the same check as the normal Event registration.

Comments

This should be my last Event Cart PR! Thank you :-)

[civibot]

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[eileenmcnaughton]

Per comments elsewhere - we can't have the same review bar for event cart because the code ahs always been kinda-alpha-y - & we have no reviewers / experts in this code - merging this based on reading the code & it making sense