This means the ACs server received an Accounting start record for a port that it hadn't received a stop record for. You usually see this when you have both exec and enable accounting turned on on the router. When a user logs in initially, an accounting start record is sent with a specific port. When that same user then goes into enable mode on the router, another accounting start record is sent with the same port (becuase it's still the same session as far as the router is concerned). ACS will put both in the Accounting logs, but the 2nd start record will show up as NAS-port reused cause there was no Accounting stop record in between them. Nothing to worry about generally.
