You can do this by checking `On Control Resolved Users Changed` event for the Client People picker. Please find more info [here][1] for Client People Picker Events.

You can try following script block to check if user is already entered in people picker.

    this.SPClientPeoplePicker.SPClientPeoplePickerDict.peoplePickerDiv_TopSpan.OnUserResolvedClientScript = function (peoplePickerId, selectedUsersInfo) {
        console.log('inside OnUserResolvedClientScript');
	    var users = selectedUsersInfo;
	    for(var i = 0; i < users.length - 1; i++){
		    if(users[users.length - 1].Key == users[i].Key){
			    alert("User already exists");
		    }
	
        }
    };


  [1]: http://ranaictiu-technicalblog.blogspot.in/2014/11/sharepoint-2013-client-side-people.html