Fix doc/schema/prefix duplicate id check

See NXS-333: some checks were failing in edition because the same feature was checked than the one being edited => duplication was always detected when leaving the docId/prefix/schemaId as is so test has been commented out, see:
https://hgpriv.nuxeo.org/nuxeo-studio/rev/c50254443ada

Needs to be put back for both creation and edition, on both client and server side (for users who would have already introduced these inconsistencies)