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Try by changing the code like this.
(defun salvameubackup() (setq ringobackupname (strcat (getvar "dwgprefix") "backup_"(getvar "dwgname"))) (if (findfile ringobackupname);check if the file exists (command "._save" ringobackupname "yes");if file exists then replace it (command "._save" ringobackupname) );if );defun
Here's what I do in a similar situation, which avoids the need to check whether the file exists: I just include the "Yes" answer in the Save command. The first time a drawing is saved that way, the command doesn't ask whether it should be replaced, the "Yes" goes in as if it were a command, and an unknown-command message goes by at the Command: prompt line, but it doesn't interfere with anything. Then for all subsequent saves under the same name, the question is asked, and the "Yes" is there to answer it.