How to add an Equal Cost Multipath Route (ECMP) to IP Routing?

Issue

• How to add an Equal Cost Multipath Route (ECMP) to IP Routing?
• How do I add an IPv4 or IPv6 multipath route?
• The RHEL kernel has CONFIG_IP_ROUTE_MULTIPATH enabled, how to use it?
• Trying to add two routes results in an error:

# ip route add DESTINATION via GATEWAY1 dev NET1 metric 1000
# ip route add DESTINATION via GATEWAY2 dev NET2 metric 1000
RTNETLINK answers: File exists