How to check if something exists in linked list in C?

i have a linked list set up like

typedef struct client_list {
    char *client_name;
    int client_socket_fd;
    struct client_list *next;
} client;

client *client_list=NULL;

before adding to the list i check if it already exists, only if it does not then i add

if (client_list==NULL || already_there(client_list, buffer)==0) {
    client_list = add_client(buffer, client_list, i);
}

int already_there(client *client_list, char *username) {
    if (client_list == NULL) return(0);
    else if (strcmp(client_list->client_name, username)==0) return(1);
    else return already_there(client_list->next, username);
}

client *add_client(char *username, client *client_list, int client_socket_fd) {
    client *new_client = (client *)malloc(sizeof(client)); 
    new_client->client_name = username;
    new_client->client_socket_fd = client_socket_fd;
    new_client->next = client_list;
    return new_client;
}

it adds the client the first time, but then the next time, always detects that the new client is already there...
somehow strcmp is always resulting true after the first time...

What's the original source of the client name you're passing in? Are you reusing that buffer?

Because your "add_client()" code never makes a copy of that string, it just uses the address of the string that gets passed in. If you reuse the original string, you'll change the data in the node on the linked list.