This third level ability of a cavalier fighter subclass allows you to mark a creature after you hit it with a melee attack on your turn. The mark lasts until the end of your next turn, until you are incapacitated, or until someone else marks the creature. 

Its this last part that confuses me. What does it mean when it says "someone else marks the creature"? Is this just in the event that you have two cavaliers in one party to ensure their features don't stack, causing confusion. OR is this implying that if some other type of "mark" is applied it also undoes the effect?

For example on the latter situation, if a ranger were to use the Hunter's Mark spell, would this undo the cavalier's Unwavering Mark? 

I think WOTC's writers may be covering themselves in case a future archetype has a similar feature that doesn't stack with that of the Cavalier. I don't see any conflict with the Hunter's Mark spell at the moment.

RAW what would be considered "someone else marking the creature" is unclear, I highly doubt Hunter's Mark would/should disable the cavalier's feature.

RAI I believe was to prevent stacking 'taunt' skills on the target such as the Rogue Swashbuckler's Panache, the Battlemaster's Distracting Strike, or the Barbarian Ancestral Guardian's Ancestral Protectors.

Quote from NathanBC >>

RAW what would be considered "someone else marking the creature" is unclear, I highly doubt Hunter's Mark would/should disable the cavalier's feature.

RAI I believe was to prevent stacking 'taunt' skills on the target such as the Rogue Swashbuckler's Panache, the Battlemaster's Distracting Strike, or the Barbarian Ancestral Guardian's Ancestral Protectors.

even simpler...

what if there were 2 cavaliers in 1 party? (A knight campaign for example)

By RAW and RAI (as I interpret it) the second mark would negate the first. Using RPG mechanics the mark is essentially a "taunt" equivalent to "Hit me or be punished for it!", If two marks worked at the same time on the same target then the target would have disadvantage no matter who they strike and RAW was put in place to prevent that for balancing.

While it is within 5 feet of you, a creature marked by you has disadvantage on any attack roll that doesn’t target you.

In addition, if a creature marked by you deals damage to anyone other than you, you can make a special melee weapon attack against the marked creature as a bonus action on your next turn. You have advantage on the attack roll, and if it hits, the attack’s weapon deals extra damage to the target equal to half your fighter level.

Do you have to be within 5 ft of the target to get the special melee weapon attack?  For example lets say I hit the target and mark it with a pole-arm from 10 ft. away.  The creature then attacks and hits the monk standing in front of me.  It clearly doesn't get disadvantage on the attack because it is not within 5 feet of me. But do I get the special melee attack bonus action on my next turn?  It is marked.  It did deal damage to someone other then me.  It isn't within 5 feet of me.  

Quote from Fairburne_Zurlo >>

While it is within 5 feet of you, a creature marked by you has disadvantage on any attack roll that doesn’t target you.

In addition, if a creature marked by you deals damage to anyone other than you, you can make a special melee weapon attack against the marked creature as a bonus action on your next turn. You have advantage on the attack roll, and if it hits, the attack’s weapon deals extra damage to the target equal to half your fighter level.

Do you have to be within 5 ft of the target to get the special melee weapon attack?  For example lets say I hit the target and mark it with a pole-arm from 10 ft. away.  The creature then attacks and hits the monk standing in front of me.  It clearly doesn't get disadvantage on the attack because it is not within 5 feet of me. But do I get the special melee attack bonus action on my next turn?  It is marked.  It did deal damage to someone other then me.  It isn't within 5 feet of me.  

The way I read it, no, you do not have to be within five feet of the target for the second part of the feature. It's a separate paragraph, there's no reason to think the five foot restriction carries over. Since the special attack is a bonus action you can take on your next turn, not a reaction, I'd say that even if the marked target is, like, twenty-five feet away, you could move to melee range and make the attack against them.

In the DMG there are optional combat rules that mention “marking” a creature. 

Mark

This option makes it easier for melee combatants to harry each other with opportunity attacks.

When a creature makes a melee attack, it can also mark its target. Until the end of the attacker’s next turn, any opportunity attack it makes against the marked target has advantage. The opportunity attack doesn’t expend the attacker’s reaction, but the attacker can’t make the attack if anything, such as the incapacitated condition or the shocking grasp spell, is preventing it from taking reactions. The attacker is limited to one opportunity attack per turn.