Juniper randomly selects one and use it. To load balance you would need to create a policy to loadbalance and apply it to the forwarding table.
By default, when there are multiple equal-cost paths to the same destination for the active route, Junos OS uses a hash algorithm to choose one of the next-hop addresses to install in the forwarding table. Whenever the set of next hops for a destination changes in any way, the next-hop address is rechosen using the hash algorithm.