There are two important differences between the earth today and the earth 4.6 billion years ago:
1. The earth itself (including the oceans, atmosphere, etc.) is *VERY* different. For example:
a - There was almost no oxygen then, there is now (oxygen is toxic to biochemicals). Oxygen is a byproduct of life itself (photosynthesis in bacteria and plants).
b - With no oxygen, there is no ozone, which is the main filter in today's atmosphere that protects from UV radiation.
c - There was far more volcanic activity ... this changes the nature of the gasses in the atmosphere. But it also means there were a lot more steam vents at the bottom of water bodies ... which is one of the most likely environments for early biochemistry.
d - Lots of other differences.
2 - There is life today. If a primitive form of biochemical life were to start evolving today ... it would quickly be seen as "lunch" for existing life forms. In other words it is *very* difficult for life to evolve on a planet in which other life forms already exist.
So it is NOT true that life would have evolved several times.