The Winterhold guard outside of Whistling Mine (Ref ID: 0005CAA8) is set up to carry two Winterhold guard shields, plus what seems to be another randomized type of shield. It's easy to see in the Creation Kit from the actor base record (GuardWinterholdSons) why there are two copies of the unique Winterhold guard shield, but I'm not sure where the third generic one is coming from. None of the three Winterhold guards in town have this problem.
As you can see in the screenshot linked below, the mine guard has two copies of the unique shield plus a hide shield.
https://postimg.cc/sBXKxW8M
As a comparison in the screenshot linked below, a town guard only has the one unique shield.
https://postimg.cc/R3Ss5y4x
Note: Not directly related, but the mine guard also has a sleep outfit while the three in town don't. Given the inventory setup, this particular guard may have originally been a direct copy/paste from a regular Stormcloak soldier.