This Property entry details every card with a Condition that prohibits you from having another card with its name on your side of the field. It is also known as side-unique. This is a side-specific variation of Field-Unique.
This means that if a "side-unique" card is face-up on any side of the field, and a new copy of that card would enter/become face-up on that side of the field, then all new copies are sent to the Graveyard as a state-based action instead. Also, a face-upSummon may not be attempted if there is already one of those monsters face-up on the field.
Additional copies can exist face-down (you can Set/Special Summon face-down additional copies); however, the moment they become face-up, they also become subject to this rule and are then sent to the Graveyard. Also, they may not be Flip Summoned if another copy is already face-up on the field.