Please Explain why I getting E11000 duplicate error

It was showing error of duplicate email but all email were different. Then I on my MongoDB server- indexes there was an email index I deleted it and then i was able to store multiple data.
Please explain what actually happened.???

email_1 dup key {email: null}

Seems you are trying to store another null value as the “email” field which is not allowed.

Check this.