As per the documentation states, the instance ID is a unique numerical identifier generated by Google Compute Engine.
You can check the documentation here:
In a technical context a unique identifier is defined as a numeric or alphanumeric string associated with a single entity within a given system.
The aim of using unique IDs is to univocally identify entities by their ID's.
However, you are asking if it's possible that this ID could be repeated, understanding by "repeated" being used in two different instances in the same moment in time, I don't think the ID can be used by 2 instances at the same time, since the documentation clearly states that the ID it's unique.
But I don't see any inconvenience on reusing the ID of an instance that was created but has been deleted and does not exist anymore (it will still be unique).
Can you specify a little bit more your use case or describe what you're trying to achieve with the instance ID?
EDITED: adding here a comment from below this answer for better reading:
To ensure the unicity of a GCE instance, for software licensing you can use an instance identity token
- A GCE instance requests a signed token from the metadata API. This token identifies the instance and is signed by Google.
- The GCE instance provides this signed token to a verification server.
- The verification server checks the signature from Google in the token, confirming the instance is who it says it is.
- The verification server authenticates the instance and provides whatever is needed to it.