I inject my own way of opening files in COC with
let g:coc_user_config = {}
let g:coc_user_config['coc.preferences.jumpCommand'] = ':SplitIfNotOpen4COC'
Then the command is defined in my lh-vim-lib plugin to search if the buffer is already opened or not. Having more control is possible.
To use the prompt already provided by COC, you may have to search in its source code.