Hi Abhishek,
Do you mean that the error throws when you are importing event registration records?
For example, you have imported two contacts before the import of event registrations, those two contacts have same name but different mailbox, let's say:
Contact 1: Clofly Mao(Fullname), clofly1@outlook.com(Email)
Contact 2: Clofly Mao(Fullname), clofly2@outlook.com(Email)
And for the event registration records excel, it has two rows:
Row 1: ER A123(Registration ID), Clofly Mao(Contact)
Row 2: ER B123(Registration ID), Clofly Mao(Contact)
However, due to there are two duplicate Clofly Mao, so it fails to set the lookup field during import process.
Is the above the situation you encountered?