duplicate entries in OpenType coverage table

Question

Some fonts have duplicate entries in an OpenType coverage table. For example, if I decode the Roboto-Regular.ttf using ttx, I find the following as part of the GDEF table:

<MarkGlyphSetsDef>
  <MarkSetTableFormat value="1"/>
  <!-- MarkSetCount=2 -->
  <Coverage index="0">
    <Glyph value="gravecomb"/>
    <Glyph value="gravecomb"/>
    <Glyph value="acutecomb"/>
    <Glyph value="acutecomb"/>
    <Glyph value="tildecomb"/>
    <Glyph value="tildecomb"/>
    <Glyph value="hookabovecomb"/>
    <Glyph value="hookabovecomb"/>
    <Glyph value="uni030F"/>
    <Glyph value="uni030F"/>
    <Glyph value="uni0483"/>
    <Glyph value="uni0484"/>
    <Glyph value="uni0485"/>
    <Glyph value="uni0486"/>
  </Coverage>
  <Coverage index="1">
    <Glyph value="cedilla"/>
    <Glyph value="ogonek"/>
    <Glyph value="dotbelowcomb"/>
    <Glyph value="dotbelowcomb"/>
    <Glyph value="uniF6C3"/>
  </Coverage>
</MarkGlyphSetsDef>

Are these valid coverage tables, or is this a bug in the Roboto font files? Since the font is widely used, I assume that the format must be accepted by font renderers. In practice, will gravecomb have coverage index 0 or 1?