Here is the updated Typescript interface with the new 'greet' method that would return a string greeting the person:
```
interface Person {
firstName?: string;
secondName?: string;
age?: number;
visits?: number;
email?: string;
greet(): string;
}
```

To greet the person by their full name and telling them how many visits they have had, the method can be implemented in class as shown in below example.
```
const person: Person = {
firstName: "Raju",
secondName: "Pandey",
age: 27,
visits: 821,
email: "RajuPandey@gmail.com",
greet() {
return `Hello ${this.firstName} ${this.secondName}, you have visited us ${this.visits} times!`;
},
};
```
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