So after you run zmzimletctl undeploy zimlet_name might check the below directories:
If it's 5.x remove: /opt/zimbra/mailboxd/webapps/service/zimlet/folder_name
If it's 6.x remove: /opt/zimbra/zimlets-deployed/folder_name & zimlets-properties/folder_name
Might run a zmprov flushCache & clear your browser's as well.
I'm assuming there's no other errors preventing it's re-deployment, check /opt/zimbra/log/mailbox.log.
Btw were going to remove that requirement/use some other logic to determine if it's allowed rather than having to manually update numbers:
Bug 40602 - Install zimlets regardless of their version number.
You also might be interested in the _dev folder trick (so you only have to refresh the web-client not re-deploy the Zimlet each time while your testing).