If this is happening, you are in 'Single ID' mode.
This mode requires you to be in 'Scan First' Shooting Mode and makes sure that only one ID is scanned per session. Each time you scan a new ID during that session, it will replace the previous ID.
If you don’t want to be in 'Single ID' mode you can turn it off under Device Settings on the Device’s page in the ELEMENTICE™ Console. See here for more.