What is required is to add a condition or validator in Jira which could be used to restrict the "same" value to be entered in a custom field while creating/editing an issue within and across projects in Jira.

Example 1

If you have filled up a custom field with a value of 123, in ticket ABC-2.
Then another user creates/edits issue ABC-3, then that user enters the same value "123" in the same field, it will detect that there is another issue with the same value "123" of the same field, then stops the issue from being created, throwing one of the options below (which can be set by the admin):

1. Not allowing the creation of the ticket and informing the Reporter, that another issue with specific value already exists.
2. Or allows the creation of the issue but it creates a link between both issues automatically.

Example 2

If you have filled up a custom field with a value of 123, in ticket ABC-2 for ProjectA
Then another user creates/edits issue ABC-3 for ProjectB, then that user enters the same value "123" in the same field, it will detect that there is another issue with the same value "123" of the same field in ProjectA, then stops the issue from being created, throwing one of the options below (which can be set by the admin):

1. Not allowing the creation of the ticket and informing the Reporter, that another issue with a specific value already exists.