Duplicate hosts are usually caused when the /var/lib/cloudera-scm-agent/uuid file has been edited or removed.

To fix this issue you can try the following example:
Let’s say:
HostA is the original/correct host with Roles on it.
HostB is the new duplicate/incorrect host.
From the Cloudera Manager UI navigate to the Host page (CM -> Hosts -> “All Hosts”)

1. Identify original UUID
Click on HostA -> Under “Details” record the “Host ID”. This is the UUID. Let’s call it “HostA-UUID” (it usually is a long alpha numeric number).

2. login to the Host from command line and navigate to var/lib/cloudera-scm-agent/

3. Stop the agent
service cloudera-scm-agent stop

4. Make a backup of the existing uuid file.

5. Change the contents of the uuid file with the original uuid (i.e HostA-UUID from step 1)
echo -n “HostA-UUID” > /var/lib/cloudera-scm-agent/uuid

For example:
In my local env my UUID/Host ID is f7b2231c-dbd0-47ff-bf09-7961080cc065:
echo –n “f7b2231c-dbd0-47ff-bf09-7961080cc065” > /var/lib/cloudera-scm-agent/uuid

6. Make sure you have not introduced a carriage return by running cat on the uuif file:
cat uuid
(your command line prompt should be on the same line as the restored uuid.
For example:
[mytestenv] cat uuid

f7b2231c-dbd0-47ff-bf09-7961080cc065[mytestenv]

7. Start the Agent:
service cloudera-scm-agent

8. From the Host page in Cloudera manager, check box HostB (the bad duplicate host with no roles) -> select the “Actions for Selected” drop down -> Delete