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Write SOQL Queries, Error of Duplication

Hi guys,
I'm having the error below and cannot move forward in Trailhead. 
Can someone help me to find out what is the issue?

User-added image

Thank you,
Pramodh KumarPramodh Kumar
Share your code.
Take a look at my code.
public class ContactSearch {
    public static list<contact> searchForContacts(string first, string postal){
        list<contact> c = new list<contact>([select id, name from contact where lastname =: first and mailingpostalCode=:postal]);
        return c;


let me know if you have any further questions

Hi @Pramodh,
I'm receiving the same error.