mount_smbfs: mount error: /Volumes/backup: File exists

Dear All,

I have a small problem to mount a remote folder which is already mounted on another foler.

I have two folders:
/Volumes/data
/Volumes/backup

With cmd+k , the remote folder is mount on /Volumes/data

Then I have a script which run rsync and it mount the same remote folder on /Volumes/backup

FIEST CASE:
When I DO NOT mount the remote folder with cmd+k, the script run well, and the remote folder is perfectly mounted on /Volumes/backup.

SECOND CASE:
But, when I previuosly mount the remote folder wiht cmd+K on /Volumes/data, and then I lunch ma script, it returns that error message (while backup folder is empty)

mount_smbfs: mount error: /Volumes/backup: File exists

Here is my mount command:
mount_smbfs -d 777 "//$DOMAIN;$USERNAME@files$NO.domain.ch$REMOTE_FOLDER/" "$LOCAL MOUNTFOLDER/"

My Question is, how can I mount twice the same remote folder on different local folder, without a such error message?

Many thank for your help