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, 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;
    }
}

 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;
    }
}