How to prevent to create multiple id on selecting multiple option?

When I choosing multiple option in select box it make more than 1 id it store in different id I want to store all selected option in 1 id.

This is my create form

<form method="post" action="">
        @csrf
        <table>
                <tr>
                    <td>Task Title : </td>
                    <td>
                        <select name="task_id" id="task_id">
                            @foreach ($tasks as $task)

                                <option value=""></option>

                            @endforeach
                        </select>
                    </td>
                </tr>
                <tr>
                    <td>Staff Name : </td>
                    <td>
                        <select name="staff_id" id="staff_id" multiple>
                            <option value=''>Select One</option>
                            @foreach ($staffs as $staff)

                                <option value=""></option>

                            @endforeach
                        </select>
                    </td>
                </tr>
                <tr>
                    <td>Done At :</td>
                    <td><input type="time" name="done_at" class="form-control"></td>
                </tr>
            <td><button class="btn btn-primary" name="submit" type="submit" value="submit" id="submit">Submit</button></td>
        </table>
    </form>

This is my store function where i storing data recieved from form

public function store(Request $request)
{
    $request->validate([
        // 'staff' => 'array|required',
        'staff_id' => 'required',
        'task_id' => 'required',
        'done_at' => 'sometimes',
    ]);
        $assignment = Assignment::create([
            'task_id' => $request->input('task_id'),
            'staff_id' => $request->input('staff_id'),
            'done_at' => 'null',
        ]);
        return redirect()->route('assignments.index', compact('assignment'))->withSuccess('Done');
}

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