Can a landlord double bill for an apartment?

[SJ Eplett]

I paid for my rental apartment through the month, but moved mid month. 

The landlord said I was responsible for the whole month.  But but moved a new tenant in mid month, directly after I moved out.

Should I have been prorated for that month? Or should I just leave this alone? 

Does anyone know if there are landlord guidelines about this situation?