yang-oid-plugin generates duplicate OIDs

If the yang-oid-plugin is executed multiple times, it writes the same OIDs again, so there are multiple for each method. This isn't legal Java 7 syntax so it causes compile errors:

target/generated-sources/sal/org/opendaylight/yang/gen/v1/urn/ietf/params/xml/ns/yang/smiv2/_if/mib/rev000614/interfaces/group/IfEntryBuilder.java:[428,9] duplicate annotation

It should just overwrite the OID so there is always one per get method.

To reproduce this, run "mvn generate-sources" in mibs-model, then examine IfEntryBuilder.java

It also happens during a normal "clean install", apparently the plugin is executed twice. Errors can be seen in the output though they don't fail the build. Other tools (such as eclipse) do fail though.