In their official docs, ServiceNow recommends the following practice:

Set completed update set on the production instance to Ignore. This state ensures the update set is not reapplied when cloning the instance.

Why would you want to do this?

To prevent the Update Sets from being pulled to Production again when you clone over your Dev instance (assuming you are pulling update sets from your Dev instance to Production).

To explain why this happens, here's what happens when you work with update sets and clone...

On your Dev instance, Update Sets that you create and complete are stored in the sys_update_set table.

Let's just say you're working with update set name 'My first update set' with a sys_id of 6a31d3973b331300bcc36da0a3efc4f5.

When you pull from your Dev instance to your Production instance, the update sets are staged in the sys_remote_update_set table.

The sys_remote_update_set record will have a reference to the 6a31d3973b331300bcc36da0a3efc4f5 sys_id in your dev instance so it doesn't pull the same update set again if it's already there.

When you commit that update set, the platform creates a sys_update_set record in production BUT with a different, new sys_id on production.

So now you have an update set named 'My first update set' on production with sys_id of 0bb291601bc28910277b33f3cc4bcb3a.

When you clone over your Dev instance, your Dev instance will then have an update set named 'My first update set' on production with sys_id of 0bb291601bc28910277b33f3cc4bcb3a.

The one with sys_id of 6a31d3973b331300bcc36da0a3efc4f5 is now gone because you cloned over it.

If you hadn't set this update set to Ignore, then when you pull for update sets from your Dev instance from production, you will pull this update set back to production as one to be committed.

It doesn't matter that there's already an update set in the Retrieved Update Set [sys_remote_update_set] list that is called 'My first update set' because there is another one with a new sys_id.

You wouldn't commit the duplicate so it's probably harmless but it makes your system messy and more prone to errors if the Update Set is there and can be accidentally committed.