On the strip and I have an 'Accepted' status with them. When I go into the Las Vegas Station building the ones downstairs react fine and one talks to me but when I go upstairs there are 2 NCR guarding a door and without provocation at all they start firing on me.

Is this normal? If not what can I do to fix?

This game is very picky about where you walk. Sometimes they'll be in the middle of a sentence saying "Stand back, this is for NCR personnel only," etc. and before you can even stop walking they're already shooting.

http://fallout.wikia.com/wiki/LVB_station

It says the ncr will shoot on sight if you dont have a high enough reputation with them.

From what I've read, you probably need to be Liked, or have Boone in your party.