id attributes MUST be unique. If you're using the metadata editor, add an "id" property to each creator entry and choose a unique value for it (rather than leaving it up to chance). Then you can add the roles at your leisure.
Valid metadata for an EPUB3 with four authors and one editor would look something like:
Code:
<dc:creator id="cre1">Author Ay</dc:creator>
<meta scheme="marc:relators" refines="#cre1" property="role">aut</meta>
<dc:creator id="cre2">Author Bee</dc:creator>
<meta scheme="marc:relators" refines="#cre2" property="role">aut</meta>
<dc:creator id="cre3">Author Cee</dc:creator>
<meta scheme="marc:relators" refines="#cre3" property="role">aut</meta>
<dc:creator id="cre4">Author Dee</dc:creator>
<meta scheme="marc:relators" refines="#cre4" property="role">aut</meta>
<dc:creator id="edt">Editor Name</dc:creator>
<meta scheme="marc:relators" refines="#edt" property="role">edt</meta>
Note that the value of the "refines" attribute (in the meta entry) matches the value of the id attribute of the dc:creator entry you widh to add a "role" to.
Use the Epubcheck plugin to verify your metadata is spec-compliant.
I don't know how calibre will respond to multiple creators/refines in the OPF. Calibre isn't 100% EPUB3 compliant. I don't know which portions of the EPUB3 metadata specifications it will support.