It seems like you've misunderstood the way that token standards should be implemented.
A master edition should never be duplicated. The issue you are facing will always be present if you attempt to create two identical master editions, for the reason described in the warning message - a duplicate master edition is typically a spoofed NFT that is used by scammers to steal funds.
It looks like the most appropriate standard in your case would be a
NonFungibleEdition, which is a 'print' of a master edition NFT. You can create multiple prints from a single master edition without encountering this error.
You might also make use of the
FungibleAsset standard, although this would more often be used to represent an in-game consumable (i.e. fuel), rather than something like a vehicle or mount.
You can read more about it here: https://docs.metaplex.com/programs/token-metadata/instructions#print-a-new-edition-from-a-master-edition.