it looks like draw:name values are supposed to be unique in ODF, even
if it's not spelled out explicitly, since it exists so the frame can be
referenced, which sort of implies that it has to be unique, so a
document where the values aren't unique can be considered invalid

    19.197.10 <draw:frame>
    The draw:name attribute specifies a name by which a <draw:frame>
element can be referenced.

So reject duplicate frames

Change-Id: I83f6d72fd969f667f0a8c2c85d2ffeeed672387a