Do I owe rent already ?

I moved on in September 20th - ok

I paid 1400$ which is the rent - the rent for what ?? September as a whole? 20 Sept to 20 Oct ? Remainder of September ??

, now the landlord is coming tomorrow to collect another 1400$? - again for what

Was the 1400 you paid for 'rent' your deposit which is generally a month's rent in advance

Why would rent be so much? I would honestly get clarification like is the rent prorated meaning you had to pay so much because you moved in in the middle of the month versus just in the beginning of the month. I think this is a rip off like did it include the deposit, water, and garbage and all of that crap. I really think you should just live month to month. If he can collect again it should state it in the lease agreement but it may not be legally enforceable. If I were you I would get help from a lawyer reading your contracts. Never sign anything you don't understand. Get help asap. 

it sounds like you do.

When does the lease say the start date is? Read it! If it says start date was Sept 1, then you owed all of September no matter when you moved in.

you might since its the first wk of the month

What is your rental agreement? If it's for the 20th of the month to the 20th of the month, then it may be a little early for the landlord to be asking for rent from October 20 to November 19, but he's not violating the agreement by asking.

If the agreement says rent is due the first of every month, then you owed October rent on October 1st, and you are late - but he owed you back 19/30ths of the $1,400 you paid for September, as you did not get the full month.

Read the agreement. If it's a verbal agreement, then you are month to month and you can then argue that rent runs from the 20th, as he has nothing in writing to say otherwise, he has no proof that he charged you pro-rated rent for September. You pay him the next $1,400 by October 20th.

I wouldn't think so if you pay month to month. Do you have a lease? Use it to remind him of this when he shows up. No lease? good luck.

You signed an agreement. WE did not read it for YOU. Try reading it. It is common to pay first and last (a total of 2 months) when you sign a lease. Why would you think that all rental leases are the same? Wow!

Your landlord collected $1,400 security. He will expect red every 30 days he'll keep your $1,400 and tell your last month and provided the apartment is undamaged you'll get it back

Read the lease. You may have rented for the entire month of September. Maybe you should have waited 10 days.