Inserting a node at the end of a double linked list in C involves creating a new node, linking it with the current tail node, updating the tail pointer to point to the new node, and updating the `next` pointer of the current tail node to point to the new node. Here is an example implementation:

```c
void insert_at_end(int data) {
    struct node *new_node = (struct node*) malloc(sizeof(struct node));
    new_node->data = data;
    new_node->next = NULL;

    if (tail == NULL) {
        // List is empty, set head and tail to new node
        new_node->prev = NULL;
        head = tail = new_node;
        return;
    }

    // Link new node with current tail node
    new_node->prev = tail;
    tail->next = new_node;

    // Update tail pointer to point to new node
    tail = new_node;
}
```

In this implementation, we create a new node and set its `data` value, `prev` pointer to the current tail node, and `next` pointer to `NULL`. We then check if the list is empty. If it is, we set the `prev` pointer of the new node to `NULL`, and set both the `head` and `tail` pointers to point to the new node. We then return, as there is nothing more to be done.

If the list is not empty, we link the new node with the current tail node by setting the `prev` pointer of the new node to point to the current tail node, and the `next` pointer of the current tail node to point to the new node. We then update the `tail` pointer to point to the new node.

Note that this implementation assumes that the `node` structure has `prev`, `next`, and `data` fields, and that the `head` and `tail` pointers point to the first and last nodes in the list, respectively.

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