Got an answer to my question from trademarkia:
You can always file for a Trademark even if someone else has it. That is your right but Our attorneys will review everything and advise you against it if there are any issues or conflicts. You could file for Facebook even by filing directly to the USPTO.
They will always accept an application, due to the 45 different classes of TM. (Goods & services) Getting it approved though is a whole different matter
They only review after you have filed. So, it's within their boundary to ask you to file, review will expose the details. Then, they decide whether to approve or not