Vlan – Juniper mirroring trunk ports

It depends. With IEEE 802.1Q specified STP, where you have one instance per trunk, STP will block ports.

With Cisco's PVST+ (Per-VLAN STP), every VLAN has a corresponding instance of STP. This means that if there are no loops within the VLANs, STP will not block ports.

MST, defined in IEEE 802.1s, works a bit differently. Instead of providing a one-to-one VLAN-STP mapping, VLANs can be grouped together into a few instances of STP. This way, if you do STP odd-even load balancing (odd VLANs over link A, even over link B) you only need two instances of STP, instead of one instance for every single VLAN. If you use MST correctly, your proposed configuration should work without any blocked links, just like PVST+, but with VLANs grouped into instances instead of working with each VLAN independently.

 family ethernet-switching and other families are mutually exclusive

You're using too many parameters under the family stanza. This extra parameter is family inet based on this:

”On this interface I have already configured IP address.”

Run user@qfx5100# delete interfaces ge-0/0/0.0 family inet. Optionally, you could delete all of ge-0/0/0.0 with user@qfx5100# delete interfaces ge-0/0/0.0 and then put your configurations back in.

When running show interfaces and show vlans, you should see this:

interfaces {
    ge-0/0/0 {
        unit 0 {
            family ethernet-switching {
                vlan {
                    members Customer1;
                }
            }
        }
    }
    ge-0/0/20 {
        unit 0 {
            family ethernet-switching {
                vlan {
                    members Customer1;
                }
            }
        }
    }    
}
vlans {
    Customer1 {
        vland-id 11;
    }
}