I just voted in the Repub primary run-off, and talked to the poll workers.
Here's the rule for NC:
You can vote in the Repub primary run-off if
1. You are registered Repub, whether or not you voted in the original primary; or
2. You are registered unaffiliated AND you voted in the original Repub primary.
(NC gives unaffiliated voters the choice of which party primary to vote in for the original primary.)
Please see Post #66.