I need help making the program not allow duplicate id's like in the picture

#include

#include

using namespace std;

struct Node $\{$

int id;

string name;

float gpa;

Node$*$ next;

$\}$;

Node$*$ createNode$($int id$,$ const string& name, float gpa$)\{$

Node$*$ newNode $=$ new Node;

newNode$->$id $=$ id;

newNode$->$name $=$ name;

newNode$->$gpa $=$ gpa;

newNode$->$next $=$ nullptr;

return newNode;

$\}$

void addNode$($Node$*$& head, int id$,$ const string& name, float gpa$)\{$

Node$*$ newNode $=$ createNode$($id$,$ name, gpa$)$;

if $($head $==$ nullptr$)\{$

head $=$ newNode;

return;

$\}$

if $($head$->$id $>$ id$)\{$

newNode$->$next $=$ head;

head $=$ newNode;

return;

$\}$

Node$*$ current $=$ head;

while $($current$->$next $!=$ nullptr && current$->$next$->$id $$ id$)\{$

current $=$ current$->$next;

$\}$

if $($current$->$next $!=$ nullptr && current$->$next$->$id $==$ id$)\{$

cout $$ "Duplicate ID found. Please enter other ID$."$ endl;

delete newNode;

return;

$\}$

newNode$->$next $=$ current$->$next;

current$->$next $=$ newNode;

$\}$

void deleteNode$($Node$*$& head, int id$)\{$

if $($head $==$ nullptr$)\{$

cout $$ "List is empty. Cannot delete." $$ endl;

return;

$\}$

if $($head$->$id $==$ id$)\{$

Node$*$ temp $=$ head;

head $=$ head$->$next;

delete temp;

return;

$\}$

Node$*$ current $=$ head;

while $($current$->$next $!=$ nullptr && current$->$next$->$id $!=$ id$)\{$

current $=$ current$->$next;

$\}$

if $($current$->$next $==$ nullptr$)\{$

cout ID not found." $$ endl;

return;

$\}$

Node$*$ temp $=$ current$->$next;

current$->$next $=$ current$->$next$->$next;

delete temp;

$\}$

void displayList$($const Node$*$ head$)\{$

if $($head $==$ nullptr$)\{$

cout $$ "List is empty." $$ endl;

return;

$\}$

const Node$*$ current $=$ head;

while $($current $!=$ nullptr$)\{$

cout ID: $"$ current$->$id Name: $"$ current$->$name GPA: $"$ current$->$gpa $$ endl;

current $=$ current$->$next;

$\}$

$\}$

void searchList$($const Node$*$ head, int id$)\{$

if $($head $==$ nullptr$)\{$

cout $$ "List is empty." $$ endl;

return;

$\}$

const Node$*$ current $=$ head;

while $($current $!=$ nullptr$)\{$

if $($current$->$id $==$ id$)\{$

cout ID: $"$ current$->$id Name: $"$ current$->$name GPA: $"$ current$->$gpa $$ endl;

return;

$\}$

current $=$ current$->$next;

$\}$

cout ID not found." $$ endl;

$\}$

void modifyNode$($Node$*$ head, int id$,$ const string& name, float gpa$)\{$

if $($head $==$ nullptr$)\{$

cout $$ "List is empty. Cannot modify." $$ endl;

return;

$\}$

if $($head$->$id $==$ id$)\{$

head$->$name $=$ name;

head$->$gpa $=$ gpa;

cout $$ "The number is modified." $$ endl;

return;

$\}$

deleteNode$($head$,$ id$)$;

addNode$($head$,$ id$,$ name, gpa$)$;

$\}$

void purgeList$($Node$*$& head$)\{$

while $($head $!=$ nullptr$)\{$

Node$*$ temp $=$ head;

head $=$ head$->$next;

delete temp;

$\}$

$\}$

int main$()\{$

Node$*$ head $=$ nullptr;

bool created $=$ false;

while $($true$)\{$

cout $$ "Enter command" $$ endl;

cout Create

$2.$ Add

$3.$ Delete

$4.$ Display

$5.$ Modify

$6.$ Purge entire list

$7.$ Search for node

$8.$ Exit the program" $$ endl;

string command;

cin $>>$ command;

if $($command $=="1")\{$

created $=$ true;

cout $$ "Linked list created." $$ endl;

$\}$ else if $($command $=="2")\{$

if $(!$created$)\{$

cout $$ "Create Linked list first." $$ endl;

continue;

$\}$

int id;

string name;

float gpa;

cout $$ "Enter ID: $"$;

cin $>>$ id;

cout $$ "Enter Name: $"$;

cin $>>$ name;

cout $$ "Enter GPA: $"$;

cin $>>$ gpa;

cout $$ "Node added." $$ endl;

addNode$($head$,$ id$,$ name, gpa$)$;

$\}$ else if $($command $=="3")\{$

if $(!$created$)\{$

cout $$ "Create Linked list first." $$ endl;

continue;

$\}$

int id;

cout $$ "Enter ID to delete: $"$;

cin $>>$ id;

deleteNode$($head$,$ id$)$;

cout $$ "Node deleted." $$ endl;

$\}$ else if $($command $=="4")\{$

if $(!$created$)\{$

cout $$ "Create Linked list first." $$ endl;

continue;

$\}$

displayList$($head$)$;

$\}$ else if $($command $=="5")\{$

if $(!$created$)\{$

cout $$ "Create Linked list first." $$ endl;

continue;

$\}$

int id;

string name;

float gpa;

cout $$ "Enter ID to modify: $"$;