If conditions on earth are perfect for life to form shouldn't have happened more than once?

Life has to start pretty simple. Chains of very basic molecules, that kinda self replicate. Those molecules would be tasty nutrients without any evolutionary self defense mechanism. So anything like that would be immediately eaten by the stablished life forms. No chance for enough self replication to evolve into anything.

Our current life tree have had many million years of evolution, we all are perfect killing machines. No proto life have a chance.

Were perfect. The conditions on earth back when life first appeared were very very different than they are now.

As for how many times it may have happened, there's no way to know, only that it happened at least once (edit: unless it happened elsewhere and came here somehow).

I don't have the answer but what an interesting question!

For all we know it did. We believe that the Earth is 4.5 billion years old but it's estimated that conditions suitable for life only appeared about 200 million years after that. Since the oldest fossils we've found are 3.7 billion years old, there is a 600 million year gap between when we think life could have formed and out earliest records of it.

There is every possibility that life formed multiple times in different environments on Earth in those first few hundred million years and then been wiped out by one of the frequent cataclysms that ravaged the early Earth. We have no way of knowing though. If life formed around a volcanic vent and then got wiped out by a meteor impact there would be no evidence it ever happened. Even if such life was wiped out by a climatic shift or something like that, there still wouldn't likely be much evidence left if any right now. The Earth's surface has been changed so much in the last 3.7 billion years, there are very few areas older than that where such fossil records from before that could be found.

And there's a good chance that other life would be chemically or structurally similar, so without DNA evidence we'd confuse it's fossils with others (see Prototaxites).
Also, maybe life does reoccur relatively frequently, but is killed by existing bacteria, viruses, bacteriophage... again, for being too chemically/structurally similar to the existing life.
- Isn't there evidence mitochondria took a different evolutionary path before they were captured by cells?
  There are also a few other really weird cellular-scale life forms I forget the names of that could be different evolutionary paths. Though I'd think the seeming hegemony of life comes from life competing in the same environments and either killing off or adapting to/with other evolutionary chains like with mitochondria. It surely wouldn't take billions or even millions of years for microscopic life to spread across Earth, so there's been plenty of time for any different upstarts to mix or kill off each other.

First of all I love this question. My suggestion is that you shuffle a deck of cards, flip them over and note their exact order, then shuffle again and note the order again then keep shuffling and checking the order until the deck resets to the original shuffled order. It's gotta happen eventually, but it might take you a while. In fact a lot of people have studied that very specific problem and there's actually really good odds that every shuffled deck you've ever held has been the only deck of that order in history. So, yes, it almost certainly has happened somewhere, but good luck finding it.

It's like when playing the lottery, if you say you're picking all your numbers in a sequence, like 1,2,3,4,5 and 6. People will tell you're crazy because sequences like that "never" happen. But the same is true for every other combination of numbers too. The sequence just makes it clearer how unlikely you are to ever pick the winning numbers.
52!
80658175170943878571660636856403766975289505440883277824000000000000
https://czep.net/weblog/52cards.html
I was fascinated for a couple years, back when flash player was coming to an end. I built a deck of cards in AS3 with the original goal of a simple single player blackjack. This was introductory research I came across that has held a fascination for me ever since.
It's gotta happen eventually, but it might take you a while.
A while is a great euphemism here... A deck of 52 cards (poker playing cards) has so many potential orders that it is said that each time someone shuffles a deck nowadays, it's really likely to get a deck order that has never been gotten before!
- That's exactly what they just said.
Found it! It had actually just fallen into the couch cushions.
There are more combinations in a deck of cards than atoms on earth.
But since we're talking about early life forming (actually chemical replicators, much simpler than a virus) let's use the card shuffling odds, but decks of cards are being shuffled in billions or trillions of places on early Earth every second for millions of years. Even a very low odds of finding a working sequence of molecules will be found geologically quickly given the amount of times done over area and time. We're pretty sure now that life began very soon once the Earth cooled down enough to allow it. What took much longer was the more complex forms of life like viruses and single cells, then even longer for multicellular.

For over three billion years, all we know of the evolution of life is from the chemical signatures it left behind, and from the genetic information of the surviving descendants. From that we can conclude that all current life arose from a common origin roughly coinciding with the first chemical signatures of living activity, and the most parsimonious explanation is that life arose on earth only once. But it’s also plausible that some of that early chemical activity was produced by forms of life that arose independently, but were displaced by ours before the emergence of multicellular organisms.

The conditions are perfect for life to thrive, and especially to evolve, we're not so sure about forming.

Actually, I'm pretty compelled by the hypothesis that Mars actually had the perfect conditions for life to form. With less of an ocean covered surface, regular rain, and constant meteor showers. Such meteors would form holes lined with random chemicals, which then get filled with water, forming a puddle. If one puddle doesn't have all the necessary components to form life, another likely will. That seems to me like a much better scenario than a sparsely diluted ocean on Earth.

Then whatever life originated on Mars might have been thrown into space by one of those meteors, and by chance, fell on Earth. There's actually evidence that such interplanetary matter transfer is possible, and has happened. That would explain why we only know of a single common ancestor, the only one that arrived here.

I think OP's question still holds, even if you think all of that happened. If there was so much life on mars and so much ejecta, why didn't multiple (differently structured, eg not DNA) rounds of life get formed on mars and transplanted to earth? Why 1x?
- Mitochondria don't use DNA, so that particular detail has already been confirmed. Doesn't mean they came from Mars, or weren't from the same precursor to DNA-based cells, but it's still interesting!
- We can't know that didn't happen. We just know that only one life form succeeded, it is very possible that others were pushed to extinction because of that.
- Then you get back to every other response on this thread, it probably did happen more than once.

I'm not convinced it's only happened once.

A new bacterium could have been created in the ocean yesterday. How would we ever know?

Assume you mean from simple building blocks instead of evolving from the reproduction of other bacteria, we could know if there were ever enough of this new bacterium for us to find and isolate it.
For one thing, if it didn't come from other living things it would not share the genetic code. Almost all organisms on earth use exactly the same translation of RNA codons to amino acids. The few exceptions are changes of just a couple of codons which had fallen to very low frequencies in the hosts' genomes.
That universal code is one of the reasons we know all life discovered to date evolved from a common ancestor.
A new bacterium, if it evolved convergently to use DNA and RNA and a 3 letter code (a big if), would not use the same translation as modern life. Even if there is some bias towards specific codons, the chance of the same core code happening twice is astronomically low.

The conditions of the early earth was very different compared to today’s earth. Particularly after photosynthesis was formed.

There is no way to know other then going back doctor who style, there's plenty of other ideas for you to consider, Stargate had the ancients, there's also a show called the 100, Homeworld games had their own thing, this could be where we evolved or this is a prison planet or a farm like in the Skyline movies or we crashed here on space Titanic, so many theories but no way to know for sure...

This takes agnosticism to an extreme. There's no way to know anything "for sure." But there are different levels of certainty, and we can work with probabilities to make reasonable conjectures. That's how science works, at least.

It shouldn't have happened at all. Abiogenisis is so hugely unlikely, it defies probability.

Even if panspermia is the leading theory for life on Earth, abiogenesis had to have happened somewhere at some point before.
I haven't had to link this in a long time. Here is the link to the relevant FAQ topic about abiogenesis from the talk.origins Usenet compilation. If you're honestly curious about the real statistics, that's a start. The cited works are obviously old but the science hasn't changed, if anything we've learned more.
Usually the strawman against abiogenesis is that a simple bacterium or virus can't just appear from nowhere, which of course is true but isn't what the science of the beginnings of life even remotely suggests. The opposite is actually true, in a world where there are no higher life forms to compete with we'd probably see all sorts of complex combinations of chemicals that eventually run across a replication process. This is the answer to OP's question, once higher life develops, the basic chemical replicators can't compete anymore. Or get absorbed into a symbiosis, as what seems to be the case with the mitochondria.
With the right conditions on other worlds (not necessarily only what Earth was like) simple life forms may be very common. We certainly now know just from recent sampling that there are planets everywhere.