However there are ways we can defined a secondary "network" where the same coin can exist in. For example bitcoin has only one blockchain and only exists there and nowhere else but it can have a second layer (such as lightning network) and same bitcoin can be used in that network.
This should not be confused with shitcoins that abuse the name of a popular coin. For example there are tokens available on shitcoin such as Ethereum that use the same name. These tokens have nothing to do with bitcoin even if they promise 1:1 swap rate and use the same name.
Same with tokens that exist on multiple shitplatforms. Just because the name is the same and exchanges swap them 1:1 doesn't mean they are linked. They are just completely separate tokens on separate shitplatforms (eg. on ETH, BNB,...) that are centralized and the centralized authority promises you a 1:1 swap ratio.