Conflict state counterparts: what's a word that means to "prevent something from being created because of pre-existing object"?

I'm writing an application logging subsystem (a software component) and I have to log system conflict events. The log is structured and all of its records are just bunches of key/value pairs, where keys denote a certain subject field/components, and values denote what actually happened. A conflict case is supposed to be a case when submitting a new object to the system must be rejected under certain circumstances. Let's say, if the system already has a user account with a certain email, then creating a new user account with exact email must be reject, and a single log record should be logged, for example:

• CONFLICT___EXISTING_OBJECT_ID = 31829
• CONFLICT___EXISTING_OBJECT_TYPE = User account
• CONFLICT___TYPE = Email conflict

The prefix word CONFLICT might be omitted as it's just a grouping word (there much more keys in the subsystem). However, I'm not sure word EXISTING denotes the already existing object that already has a unique email. I was thinking of some alternatives like but I can't choose a good one:

• CONFLICT___BLOCKING_OBJECT_ID - blocking seems to have slightly another meaning
• CONFLICT___OLD_OBJECT_ID - this is even more confusing, because the existing object is not qualified as old one, and no time-related stuff is supposed to be used here
• CONFLICT___CONFLICTING_OBJECT_ID - from my understanding, the word conflicting should be applied to the object being submitted and then rejected for whatever reason, and this word should not be used for the already existing object.
• CONFLICT___INHIBITOR_OBJECT_ID - in some sense, it really can describe the existing object, but it's not a word to mean a "conflict".

Is there an adjective to describe an existing object that prevents a new object from being submitted (thus the latter is conflicting [from my understanding]) for some reasons in the context of some conflict in English?

You could use the verb obstructing

Cambridge dictionary

to block or get in the way of something or someone, or to prevent something from happening or progressing by causing difficulties

Thus an existing object is obstructing a new object from going through.

or the verb preclude

dictionary.com

1. to prevent the presence, existence, or occurrence of; make impossible

The existing object precludes the creation of a new one