The MX at this time does not support ECMP so you can only enter 1 route for each destination. However in your example you might run into an issue where you try to add an aggregated subnet 10.10.0.0/16 for where you already have a local VLAN 10.10.51.0/24 which is more specific. Normally you can have routes to less specific subnets and routes via several local subnets. But I would try to add a more specific route than the 10.10.0.0/16 first or remove the 10.10.51.0/24 subnets just as a test and see if you can add the aggregate route then.  Usually you can also see the error in a red banner on the top of the dashboard page.  But sometimes these can be hidden and show themselves when you push via API, ... View more