The Bluetooth v4.0 specification says:
A service may have multiple characteristic definitions with the same Characteristic UUID. 
Are there any GATT services that have multiple characteristics with the same UUID?
What is the use case for this?
 (Vol 3, Part G, 3.3.1 "Characteristic Declaration")
Best How To :
Normally it won't be that useful… A simple use case can be:
Like if I have a BTLE device try to measure the Magnetic Flux Density difference between two points in space and also provide the flux density on both points to user, instead of publishing two Environmental Sensing services, it's making more sense to publish one Environmental Sensing service with two "Magnetic Flux Density - 2D" characteristics.