There can only be one coefficient before a compound. When you are writing out and balancing equations, only one number can be put in front of each compound. There is only one set of numbers that can fit an equation (if you exclude the multiples of that set).
Here are some examples:
combination of mercury and oxygen
(can only be 2, 1, 2 if you exclude multiples)
complete neutralization of phosphoric acid with sodium hydroxide
(can only be 1, 3, 1, 3 if you exclude multiples)