trigger CustomerTrigger on Customer__c (before update){
list<test__c> updateTest=new list<test__c>();
if(trigger.isupdate && trigger.isbefore){
    for(customer__c cust:trigger.new){
        test__c t=new test__c(id=cust.test__c,salary__c=cust.salary__c,phone__c=cust.phone__C);
        updateTest.add(t);
    }
}

ERROR: CustomerTrigger: execution of BeforeUpdatecaused by: System.ListException: Duplicate id in list: a1O28000007ZypIEASTrigger.CustomerTrigger: line 10, column 1