Redundancy is the duplication of critical components in the data centre system, for the purpose of increasing reliability. Within the data centre world, the greater the redundancy, the smaller the likely hood of experiencing downtime.
In a data centre system if you require 10 UPS units for normal operation, in a N+1 system you would have your 10 UPS plus 1 additional unit to reduce chance of downtime. In order to run the same 10 unit data centre in a 2N system you would need 20 UPS units 10 for normal operation and 10 more for backup. In a 2N system, unlike N+1, all equipment needs to run separetly with no single failure points.
Have a look at our Data Centre Tier guide for redundancy requirements based on Tier levels.
Data Center Redundancy