My cat that's been missing for 7 MONTHS just came home!! Buy I have a new kitten HELP PLEASEE?

My male cat who is 6 years old & not neutered went missing 7 months ago in march he was an outdoor cat & always came home to eat & sleep in the day then he'd be out for a few hours sometimes even a few days but he always came back within a week.. so 3 days ago after giving up searching for him after... show more My male cat who is 6 years old & not neutered went missing 7 months ago in march he was an outdoor cat & always came home to eat & sleep in the day then he'd be out for a few hours sometimes even a few days but he always came back within a week.. so 3 days ago after giving up searching for him after 7 months! I had decided to get a new kitten he's been settling in perfectly & he's 10 weeks old.. but about an hour ago I had left my window open & my old cat came back fat! well fed & clean & looked after so he obviously found another home! I left my window open for the best part of 7 months & he never ever came home until today 3 days after getting my new kitten! Its crazy! I put my old cat in the living room fed him he was purring & was sitting between my legs... I put him on a furry blanket & hes fallen asleep.. it's so strange its like he never left! My new kitten is now locked in my bedroom with me I'm obviously keeping them apart because I know my old cat will kill him! He was a very anti social cat always hissed at visitors & my moms dog etc thats why im shocked he found a new home as he's not neutered .. but now I don't know what to do! My new kitten has a vet appointment next week to get neutered & is forever going to be an indoor cat I learnt from my mistakes with my old cat..
Update: I love my old cat but when he wakes up I'm sure hes going to go out again through the window & probably go back to his other owner I don't know if he will be back and forth eating from both of us & I don't want that I want a loyal cat that has one owner..
Update 2: but if my cat decides to stay with me n come in & out I'm sure he'll never get along with my new kitten I asked my mom & she thinks one of my neighbours stole my cat & only just let him out because they saw I have a new kitten but I doubt that very much because he has old scabs on his head where... show more but if my cat decides to stay with me n come in & out I'm sure he'll never get along with my new kitten I asked my mom & she thinks one of my neighbours stole my cat & only just let him out because they saw I have a new kitten but I doubt that very much because he has old scabs on his head where he's been fighting other cats outside I presume so why would he choose to come back after 7 months!
Update 3: I don't know what to do ideally id like to have both cats as indoor cats get my old cat neutered & live happily ever after but I doubt my old cat will want to be an indoor cat & I don't even know if he wants to stay with me HELP I don't know what to do!!!
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