Keep 1 Char - Linked List

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

typedef struct Node
{
    char value;
    struct Node* next;
} node;

node* create_node(char data)
{
    node* temp = (node*)malloc(sizeof(node));

    temp->value = data;
    temp->next = NULL;

    return temp;
}

void insert(node** head, char value)
{
    node* link = create_node(value);

    if (*head == NULL)
    {
        *head = link;
        return;
    }

    node* temp = *head;

    while (temp->next != NULL)
    {
        temp = temp->next;
    }

    temp->next = link;
}

void print_list(node* head)
{
    node* temp = head;

    if (head == NULL)
    {
        printf("List is empty.\n");
        return;
    }

    while (temp != NULL)
    {
        printf("%c-> ", temp->value);
        temp = temp->next;
    }
    printf("\n");
}

void delete_after(node* temp)
{
    if (temp == NULL || temp->next == NULL)
        return;

    node* delete_node = temp->next;
    temp->next = temp->next->next;
    free(delete_node);
}

void keep_one(node** head, char c)
{
    node* temp = *head;
    int counter = 0;

    if (*head == NULL) return;

    if ((*head)->value == c)
        counter++;

    while (temp->next != NULL)
    {
        if (temp->next->value == c)
        {
            if (counter < 1)
            {
                counter++;
            }

            else
            {
                delete_after(temp);
                counter++;
            }
        }

        if (temp->next)
        {
            if (temp->next->value == c)
            {
                if (counter <= 1)
                    temp = temp->next;
            }

            else
                temp = temp->next;
        }
    }
}

void removeDupList(node** head)
{
    node* temp = *head;
    while (temp != NULL)
    {
        keep_one(&(*head), temp->value);
        temp = temp->next;
    }
}

void removeDupArrayList(node** arr, int size)
{
    for (int i = 0; i < size; i++)
    {
        removeDupList(&(arr[i]));
    }
}

void main()
{
    node* head = NULL;
    node* head1 = NULL;
    node* head2 = NULL;
    node* head3 = NULL;

    insert(&head, 'a');
    insert(&head, 'c');
    insert(&head, 'a');
    insert(&head, 'b');
    insert(&head, 'b');
    insert(&head, 'a');

    insert(&head1, 'c');
    insert(&head1, 'd');
    insert(&head1, 'c');

    insert(&head2, 'a');

    insert(&head3, 'e');
    insert(&head3, 'e');
    insert(&head3, 'e');

    printf("\n\------Old Lists ------\n\n");

    print_list(head);
    print_list(head1);
    print_list(head2);
    print_list(head3);

    node* arr[4] = { head, head1, head2, head3 };

    removeDupArrayList(arr, 4);

    printf("\n\n------ New Lists ------\n\n");

    print_list(head);
    print_list(head1);
    print_list(head2);
    print_list(head3);
}