If (and when) an address collision will happen it doesn't matter if I have the second private key. With only one I reach it, right?

If you have a private key that generates an address, you can spend from that address. Any and every key that generates the address will work.

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If (and when) an address collision will happen it doesn't matter if I have the second private key. With only one I reach it, right?

All suitable keys must be identical. But do not worry, this most likely will never happen

Keys do not have to be identical. they just need to generate the same address. and for each address there are about 2^96 keys that will work.