Can A Landlord Charge Two Tenants Rent For The Same Apartment?

If your lease was over, then he was charging from only one tenant as per the policy. But at the same time, you both were paying the rent which could be taken into courts against the landlord.

Absolutely NOT.  If your lease was up, and he rented it out, then he charges you for the rent that was owed on your contract.  However, if you left and your lease was still running, and he immediately rented the space (he has a duty to mitigate damages), then he can not charge you for that rent...however, he may charge you for the reasonable fees of having to find a tenant to cover the remainder of your lease.  If however, he gets LESS rent than what was on your lease, he can get from you the difference.  For example:
If your lease ends in June1st, and you vacated in April 1st, and your rent was $1500, but the new tenant only agreed to pay $1200, and took possession at the beginning of April, then your landlord can get $300 from you for the months of April, May, June plus the reasonable cost of seeking a new tenant.  In other words, the landlord can NOT charge you full rent owed for those three months and also charge the new tenant.